15/04/2016 12:06 BST | Updated 16/04/2017 06:12 BST

Boris Johnson Spearheads 'Brexit Blitz' As EU Campaign Officially Begins

Brexit will give the NHS a multi-billion pound boost, London mayor Boris Johnson will insist as he kicks off a series of rallies on the first day of the official referendum campaign.

The Vote Leave organisation, backed by Mr Johnson and Cabinet heavyweight Michael Gove, moved the NHS centre stage in the campaign as it claimed a large chunk of the UK's £10.6 billion net contribution to Brussels could be diverted to medical care if Britain quits the EU.

Mr Johnson will spearhead a "Brexit blitz" of speeches across the UK with an address in Manchester, ahead of weekend events in Leeds and Newcastle.

His rallying cry speech comes on the first official day of the campaign which sees rules governing spending and other areas come into play ahead of the June 23 vote.

The cranking-up of the campaign came as Labour former chancellor Alistair Darling was set to accuse Brexit backers of offering "Project Fantasy", as he warns that leaving would threaten Britain's economy.

"As a country whose long-term economic recovery relies on the health of our exports and our regulatory environment being attractive to global investors, it would be a colossal surrender of power to walk away from our largest trading partner, increase uncertainty, erect trade barriers and diminish our influence," he will say.

On the withdrawal side, Vote Leave's decision to highlight projected funding problems within the NHS is bound to heighten internal Tory tensions, as Prime Minister David Cameron has strongly defended his record on the health service.

The campaign group points to predictions showing the NHS could face a shortfall in funding of £12.3 billion by 2020-2021 as reason to divert resources from the EU to healthcare.

"The UK's net contribution to the EU is £10.6 billion. This suggests the NHS deficit could nearly be eliminated if we vote leave and we choose to spend our money on our priorities like the NHS," a Vote Leave spokesman said.

Vote Leave chairwoman Gisela Stuart, a Labour MP, insisted money needed to be switched from Brussels to the NHS.

She said: "The NHS is facing a crisis due to growing demand for healthcare and a squeeze on funding. This means that patients are not always getting the treatment that they need, when they need it.

"If we vote leave we will be able to stop handing over £350 million a week to Brussels and we will be able to instead spend our money on our priorities like the NHS."

Justice Secretary Mr Gove is expected to echo the NHS warnings during a speech in Nottingham, as fellow leading Leave figures Commons Leader Chris Grayling and Northern Ireland Secretary Theresa Villiers also step-up the Brexit campaign in the coming days.

Prime Minister David Cameron's official spokeswoman said: "A strong NHS needs a strong economy and the Prime Minister has been very clear that our economy is stronger in the EU.

"You have to look at some of those who have expertise and experience of working in the NHS, like Sir David Nicholson, the former chief executive of the health service, who has said that leaving the EU would damage the economy and mean less money for the NHS."