13/03/2015 07:16 GMT | Updated 13/04/2015 05:59 BST

Stephen O'Brien, The Tory MP Turned UN Humanitarian Chief Swapping Westminster For New York

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Stephen O'Brien had his campaign literature designed and ready to go. Fortunately the printers had not rolled before the call came from the United Nations telling him to stop the presses.

Rather than returning to Westminster after May's election, the 57-year-old Tory MP will be heading to New York as the new UN Under-Secretary-General for Humanitarian Affairs and Emergency Relief Coordinator. And he is very pleased about it. "I was absolutely thrilled, naturally a little taken aback and of course delighted," he says.

"It's a process which attracts a lot of international expressions of interest and candidates," he observes. "And so the final process only really took place last week and then there was a notification which took place in the last few days and the announcement was made mid-day Monday."

During the interview with The Huffington Post in parliament's busy Portcullis House building, another Tory MP wanders over to congratulate O'Brien on his new job. His phone, O'Brien says, is getting a bit hot from all the phone calls and messages, including from the prime minister, he has received.

Announcing O'Brien's appointment to the top UN humanitarian job, Ban Ki-moon hailed the former international development minister's "extensive experience" in the field. In Westminster, this was widely read as a reference to one of the rejected candidates.

David Cameron's first choice for the role, which is earmarked for a Briton, was said to be Andrew Lansley. However the UN secretary general was reportedly unimpressed with the former health secretary's lack of development experience and chose O'Brien instead.

"Well, the process itself didn’t disclose who the other candidates were, so I was never privy to that," O'Brien says diplomatically when asked whether he has spoken to Lansley since beating him to the job. "Of course I was able to read the speculation like everybody else."

"I have known Andrew for many, many years. I worked extremely closely with him, not least at the time we were both in the shadow health team together where I was focusing a lot on social care.

"And so of course it's always slightly difficult where there is a position were only one can end up being appointed. But I wish Andrew well. And I very much hope his talents will be used, of course they will be available to others. I am absolutely sure he will continue to make a contribution in public life as he always has."

David Cameron with UN secretary general Ban Ki-moon

As for O'Brien, his interest in humanitarian issues goes back to when he was at Cambridge. After university, in 1979, he and two friends travelled across the Sahara to investigate how malaria was was managing to cross the natural border and reach north to Algiers. Mosquitos, it turned out, were hitching lifts in good lorries. "If you have good evidence huge political will and you can marshal the right resources you can make a phenomenal difference to not only saving lives, but improving lives. So that has been very much theme of my interest over many, many years," he says.

As well as his stint as a development minister between 2010 and 2012, O'Brien also points to his time as an executive at industrial company Redland, which he says involved coordinating complex manufacturing operations across 35 countries.

The UN job, he says, is to be the team member who coordinates the delivery of humanitarian relief to he most needy people in the world be that in areas of conflict, in response to disasters and other emergencies.

"That’s the task, that’s the substance of the job. But of course the co-ordinator is not the person who delivers on the ground. There are many, many brilliant un professionals and agencies, many brilliant NGOs with enormous experience and many other players. All the players need bringing together so they can all play to their best strengths."

O'Brien is swapping Westminster for Manhattan

O'Brien is replacing another Briton in the UN job - Baroness Valarie Amos. He is warm in his praise for the Labour peer and former development secretary. "It's again one of the great privileges to be able to follow somebody who has filled the role with such distinction and has been so energetic and successful," he says. "I have very big boots to fill."

It was during O'Brien's time as a minister that this coalition government that it delivered on Cameron's pledge to spend 0.7% of Britain's Gross national income (GNI) on overseas development aid (ODA). It is a commitment that a lot of his fellow Tory MPs are unhappy with. "I am very proud," he says, however, of keeping the promise, which is now enshrined in law.

He says development spending is not only a "moral imperative" but is in the interests of the wealthier donor countries. However he recognises not everyone is convinced. "There are plenty of people who don’t agree with it. But they weren’t in the majority"

"It's healthy democracy," he says. "You never want people not to raise objections if they have them and scrutinize very carefully. There will always be some people you do not persuade, but what is clear here is on a policy that did not start with overwhelming favour the arguments have won the day."

Earlier this week the Commons approved a Bill that made the 0.7% aid spend a legal requirement and became the first G7 country to meet the UN's 45-year-old target. "Nobody goes to prison," O'Brien acknowledges of the consequence of a future government failing to stick to the spending. "But if you don’t do it you do get a sense of being really held up to the mirror."

Many Conservative MPs do not like the ring-fencing of the aid budget. They question why at a time of public spending cuts money is being sent abroad. The conservative press is equally unimpressed. The most recent complaint is that it is the British defence budget that should be protected, not foreign aid. Just as the UN wants countries to spend 0.7% of their GNI on aid, Nato wants its members to keep defence spending at 2%. "I am very sympathetic to the idea that 2% spending on defence could be ring-fenced," O'Brien says. But argues development aid is different than other priorities.

Military spending, he says, is a "domestic" variable in the same bucket as education or health. "These are matters for us to settle as governments in democracies, to prioritise how we dispense hard won and hard earned taxpayers money." The commitment to development aid, O'Brien argues, is above that.

"There is a big distinction between the two issues," he says. "ODA is an externally defined and accountable term therefore it is an expression of intent."

He also insists that aid money is vital to the security of Britain and other Western countries. "Where there is no hope, there tends to be the possibility of people exploiting that hopelessness," he says. "There are massive links between [security] and there being extreme poverty, poverty which there seems to be no escape from, resource constraints even climate change damage which is beginning to have the biggest effect on poorest people."

O'Brien in his office following his election to parliament in 1999

O'Brien, who was born in Tanzania and educated in Mombasa before moving to Britain, has been the MP for rural Eddisbury in the North West of England since 1999. His healthy 13,255 vote majority will now be someone else's to defend in under two months time at the May 7 general election. "There was no way of making the timing a nice fit," he says of having to tell his local party he was off to Manhattan rather than setting about door-knocking for votes. "It has been quite sudden decision for my constituents and in particular for my Conservative association who must now find a successor candidate.

"I will find it quite a wrench. I have never been in any doubt about the enormous privilege it is to have a seat in the Commons and to participate in parliamentary work."

Polls show the country is heading towards a second hung parliament. However O'Brien is confident Cameron can still win a overall majority. "I very much hope he will succeed over the campaign in bringing in a result and we will wake up on May 8 with an overall Conservative majority with David Cameron as prime minister of full Conservative government," he says.

He adds, when pressed on the this unlikely scenario: "My personal opinion is, yes, there is everything to fight for in a campaign, there always is, there is a very real prospect of the Conservatives prevailing."

But it seems clear O'Brien's head is, understandably, already two thousand miles away at the UN rather than on the upcoming election here at home. "I will adjust extremely quickly. There is such a lot do there won't be time to contemplate the past, you'll simply have to get on with it," he says.

He adds: "I must ask someone to help me find an appropriate small flat in New York."