General election 2017: David Cameron on need to stop 'extreme Brexit'
A strong election victory would allow Theresa May to "stand up to people who want an extreme Brexit either here or in Brussels", David Cameron says.
The former PM, who quit after the UK voted to leave the EU last year, was on the campaign trail in Cheshire.
He said the Conservatives must "win and win well" to strengthen his successor's hand in the Brexit negotiations.
But another ex-PM, Gordon Brown, warned giving her a "free hand" would put the car industry and manufacturing at risk.
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Mr Cameron, who as prime minister called the referendum but campaigned for a Remain vote, was making a rare public appearance in Nantwich, ahead of the 8 June general election.
He said Jeremy Corbyn would make a "terrible" prime minister and that it was important that Theresa May won "the biggest possible majority" so she could deliver the "best possible Brexit deal".
"It's so important... that the Conservatives win and win well, so Theresa can negotiate that Brexit deal, so she can stand up to people who want an extreme Brexit either here or in Brussels."
Of his successor, he said: "I want her to have the biggest possible majority so she can deliver the best possible deal. The election, in a way, is a straight choice between her continuing as prime minister, with that important work, and Labour, frankly, who've put up a candidate who is not cut out to be prime minister, who would make a terrible prime minister."
Asked on LBC radio whether she had called the election to be able to stand up to those seeking an extreme Brexit, perhaps in her own party, Mrs May said: "The reason I called the election was because I think we need the security, the stability for five years of greater certainty that can take us through Brexit and beyond."
She said her predecessor was "absolutely right" to say the election was crucial adding it was about "ensuring we've got a strong negotiating hand".
Pressed over some of Mrs May's policies - such as a plan to create more grammar schools, something he opposed, Mr Cameron said he was happy to support the Conservative manifesto and the party's candidates.
In 2015 Mr Cameron described then Labour leader Ed Miliband's proposal for a energy price cap as "Marxist", but he said he understood why Mrs May wanted to bring in an energy price cap to help "hard-working people" and keep bills down.
He argued that Labour had "deserted traditional, working families who want to improve their situation - get a decent job, pay less in tax, face lower bills - and it's the Conservative party standing up for them".
But in Coventry, another former prime minister, Labour's Gordon Brown, warned against giving Mrs May "carte blanche" in Brexit negotiations, when she had yet to reveal her hand.
He said: "She is not telling us what her hand is in these negotiations and it is to be at the cost of manufacturing, at the cost of the car industry and at the cost of jobs if we are not told what we are voting for on June the 8th. She wants a free hand, she wants us to write her a blank cheque."
He also said the Conservatives' record on the NHS "shamed" Britain, adding that Mrs May risked "leaving the country more socially divided than ever unless there is a change in policy".
Speaking in front of a Harrier jump jet at Coventry University, Mr Brown said: "When Mrs May says she is for those people just getting by, the Conservative Party are walking by on the other side and ignoring these needs."