Former foreign secretary William Hague has issued a stark warning that the next Conservative leader can only survive if they have a clear plan to end the deadlock over Brexit.
Lord Hague expressed concern that not all the leading candidates in the contest are ready for what will “hit them” if they succeed in the race to succeed Theresa May.
His warning came as the remaining contenders face the first official hustings of Tory MPs, starting on Tuesday, after nominations closed on Monday.
Ten candidates will go into the first round of voting on Thursday after pro-Remain former minister Sam Gyimah pulled out admitting he had been unable to build sufficient support.
Writing in the Daily Telegraph, Lord Hague said that without a proper plan for dealing with Brexit, the next prime minister would head a “sandcastle administration” which would quickly crumble.
“It is by no means clear as things stand that all of the leading candidates have that plan in their head, and are ready for what is going to hit them if they win the leadership,” he wrote.
“The risk is that they will make tactical decisions and statements in the opening hours that will progressively enfeeble them over the subsequent weeks.”
Lord Hague did not name any names, but his comments are likely to be interpreted as an attack on Boris Johnson, who has been criticised for his lack of detailed planning.
They came as Rory Stewart hit out at rival candidates, accusing them of racking up a series of “reckless” tax and spending pledges they would be unable to keep in office.
The International Development Secretary, who launches his campaign on Tuesday, said the “eye-watering” cost of his rivals’ promises risked undermining the party’s reputation for economic prudence.
Meanwhile, Andrea Leadsom will launch her campaign with a promise to take action on the climate change “emergency”.
Mr Stewart singled out Mr Johnson - the clear frontrunner in the contest - as well as Dominic Raab, Michael Gove, Jeremy Hunt and Sajid Javid for their promised tax cuts.
He said that between them, they had run up spending pledges totalling £84 billion.
“We simply cannot make spending and tax cut promises that we can’t keep,” he said.
“We cannot criticise Jeremy Corbyn for reckless spending pledges if we start doing the same ourselves. Cheap electoral bribes could cost us dear.”
According to figures released by Mr Stewart’s campaign team, Mr Raab is the biggest spender so far with £38.2 billion of tax cuts promised.
They include raising the national insurance threshold to £12,500, scrapping stamp duty on homes under £500,000, and a 5p cut in the basic rate of income tax.
He was followed by Mr Gove, whose promise to scrap VAT and replace it with a lower and simpler sales tax was put at £20 billion.
Mr Johnson’s plan to raise the 40% tax threshold from £50,000 to £80,000 was said to cost £14.1 billion, Mr Hunt’s promise to cut corporation tax to 12.5% was put at £11 billion, and Mr Javid’s suggestion he could scrap the top rate of tax was put at £700,000.
His attack came as Mr Gove sought to get his campaign back on track after his admission that he took cocaine before entering politics, with a series of swipes at Mr Johnson - including his tax promise.
At his launch on Monday, the Environment Secretary said he would never use the tax system to “give the already wealthy another tax cut” while taunting Mr Johnson for pulling out of the last leadership race in 2016.
Mr Raab and Health Secretary Matt Hancock also criticised Mr Johnson’s proposals.
But Mr Johnson received the backing of former Conservative leader Iain Duncan Smith, who said the former foreign secretary was the “most likely” candidate to deliver Brexit by October 31.
Writing in the Daily Telegraph, he added: “Boris is, as he showed in London, capable of also capturing a mood of optimism for too long missing in our message to the electorate.”
At her launch, Mrs Leadsom, the former leader of the House, will present herself as the “optimistic yet realistic Brexiteer” who can heal the divisions in the country.
She will say moving to a carbon-neutral economy is not only right for the planet and for future generations, but also offers the chance to develop new clean technologies which could rival the UK financial services sector in size and stature.
She will outline her ambitions for a major expansion in housebuilding, to help young people get a foot on the housing ladder while providing new opportunities for those looking to downsize.
“Our party has thrived in the past when it has governed as a champion of the people, providing freedom of choice and opportunity, a strong economy and global leadership,” she is expected to say.