Former foreign secretary William Hague has warned that the next Conservative leader can only succeed if they have a clear plan to end the Brexit impasse.
His comments came as the final, 10-strong leadership line-up was revealed, with former universities minister Sam Gyimah dropping out of the race last night after failing to secure the backing of eight MPs.
The remaining candidates will go into the first round of voting on Thursday, where they must each secure at least 17 votes.
But 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.
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.
It isn't the first clash between the candidates, with Michael Gove hitting out at Boris Johnson during his campaign launch yesterday.
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.