Former prime minister Sir John Major has lashed out at "disloyal" Tory MPs and ministers as he urged them to back Theresa May - but suggested she must take radical action to win over voters by increasing public spending and reviewing controversial welfare changes.
The ex-Tory leader, whose own premiership was marred by infighting and plotting, said he had viewed the turmoil in the Conservative Party with "increasing dismay".
Alongside his appeal for unity, Sir John called for radical action on Mrs May's social justice agenda to "win back hearts and minds" or risk the prospect of "neo-Marxist" Jeremy Corbyn taking the keys to Number 10.
Sir John said the Government should review Universal Credit, the welfare shake-up which replaces up to six different benefits with a single payment.
More than a dozen Tory MPs have urged ministers to pause the roll-out of the policy amid concerns claimants could face delays in receiving money.
Sir John said the system was "theoretically impeccable" but "operationally messy, socially unfair and unforgiving".
Writing in the Mail on Sunday, he said: "It is time for the Conservative Party to show its heart again, which is all too often concealed by its financial prudence. We are not living in normal times and must challenge innate conservative caution."
Calling for further action on housing and education, Sir John urged Chancellor Philip Hammond to ease up on austerity.
Sir John suggested that the Government could take advantage of low borrowing costs to fund investment in the country's future.
"We must persuade the Treasury that - while the cost of long-term borrowing is low - there is an opportunity to vastly accelerate public development of infrastructure and, in particular, housing.
"Useful initiatives have been announced but we need to go further. If this increases public debt we should - and could - accept that (as I believe the markets will) provided annual revenue expenditure is kept under control."
Sir John was scathing about those agitating for a change in Tory leadership behind the scenes.
He said he was "saddened to see the news dominated by those who have been driven by their own personal agenda", adding "their conduct has undermined their own party, their own Prime Minister and their own Government".
The comments will be viewed as a slapdown to Foreign Secretary Boris Johnson, whose activities overshadowed much of the Tory conference until Mrs May's ill-fated speech.
"The country has had enough of the self-absorbed and, frankly, disloyal behaviour we have witnessed over recent weeks," Sir John said.
"It is time for the individuals concerned - both in Parliament and in Government - to focus their minds instead on the needs of the British people, rather than on their own personal ambition."