David Cameron has said Theresa May needs to win a strong majority in the General Election so she can stand up to people who want an "extreme Brexit".
The former prime minister, campaigning in Crewe and Nantwich, said that he wanted his successor to "win and win well".
"This is one of the most defining elections I can remember where it's so important that the Conservatives win and win well, so Theresa can negotiate that Brexit deal and stand up to people who want an extreme Brexit, either here or in Brussels," he told ITV News.
"But it's important that the Labour Party don't do well because they put up a candidate for prime minister who is completely unsuited to do the job."
Mr Cameron insisted that he had no regrets about calling last year's referendum, even though it cost him his premiership when the result went against him.
"I think it was right to hold that referendum but when that referendum result was not the one I recommended I recognised the country needed fresh leadership to take it forward, someone who had the passion and the vision to drive that through. That's what Theresa is doing," he said.
"The bigger mandate she gets, the better it is for our country because she's got to deliver that deal and I'm happy to help her do that by helping her to get the biggest majority she can."
Mr Cameron side-stepped questions about Mrs May's plans for more grammar schools - a policy he opposed in office.
Pressed as to whether he was now happy to support Mrs May's proposals, he said: "Look, I'm a strong Conservative and I want the Conservative team to win."
He also said he understood why Mrs May was proposing a cap on energy prices – an idea which he denounced as "Marxist" when former Labour leader Ed Miliband put forward a similar scheme.
"I completely understand why Theresa has opted for this policy. There's a lot of frustration about high energy prices and she wants to help hardworking people to keep bills down," he said.
Appearing later on a LBC radio phone-in, Mrs May rejected the suggestion she had called the election to strengthen her hand against people in her own party who wanted an "extreme Brexit".
"No, the reason I called the election was because we need the security, the stability for five years of greater certainty that will take us through Brexit and beyond," she said.
"It is about ensuring we have got a strong negotiating hand. Every vote for me and my team will strengthen the UK's hand negotiating that Brexit."
Former Ukip leader Nigel Farage, speaking at the launch of a book by Bill Etheridge called Putting Great back into Britain, told those gathered in a Westminster pub that it has "not necessarily been the easiest couple of weeks" for the party.
Mr Farage said: "I love David Cameron - no one has done more for Ukip or my career than David Cameron.
"He was at it today wasn't he, he was in Nantwich on the streets campaigning and he said 'yes, it is very important that Mrs May gets a big majority so we can we stop people in this country, in our party, who want an extreme Brexit'.
"The cat is out of the bag. She wants a big majority so she can soften the position in Brussels and we basically finish up being half in, half out.
"And I can't think of a better or stronger incentive for people to vote Ukip."
Toasting the publication of the latest work by Mr Etheridge, Mr Farage also called for a glass to be raised to Mr Cameron.