Conservative leadership hopeful Rory Stewart has accused rivals, including Boris Johnson, of "misleading" voters about the prospects of negotiating a new Brexit deal.
The international development secretary said that other contenders in the race to replace Theresa May were not being "honest" about their plans, which he said amount to an offer of a no-deal departure from the EU on October 31.
And he warned of the danger of "collapsing the country" and forcing a "disastrous" general election if the Conservative Party chooses a new leader who claims to be able to deliver a new deal by the Halloween deadline.
Mr Stewart is calling for a citizens' assembly process to build consensus behind the deal secured by Ms May with Brussels last November.
Speaking to BBC Radio 5 Live's Emma Barnett, Mr Stewart said: "There is only one thing anybody can do - and this is where people are not being honest. You can't get no-deal through parliament and you can't get another deal from Europe. This is the deal we have got. We got 270 votes for it, in the end this is about getting 45 more MPs to vote for this deal."
He added: If we are to hold the country and the party together, we are going to have to start being very, very careful with being straight and that means not setting expectations we can't meet.
"The only way we are going to avoid tumbling into a disastrous election or collapsing this country is if we avoid politicians pretending they are going to get a better deal from Brussels - which they won't - or pretending they will get a new deal by October - which they won't - or pretending that they can take a no-deal through - which they won't."
Leadership candidates like Mr Johnson who claim to be able to renegotiate the EU withdrawal agreement by October are displaying a "lack of realism" and "misleading themselves and others", he said, adding: "That is going to be catastrophic."
"Let's take a candidate like Boris," said Mr Stewart. "He is saying he is going to go to Europe and negotiate a new deal by October 31 and if he doesn't get a deal before October 31 he is going to go no-deal.
"Anyone who knows anything about Europe can assure you there is not the slightest hope of getting a new deal through Europe by October 31. Not a hope.
"Any leadership candidate pretending they are going to go to Brussels and get another deal simply doesn't understand Brussels, hasn't been following the news, doesn't understand what's happening in Europe and doesn't understand that the European position is very clear.
"Anyone offering that is offering no-deal, is trying to get no-deal through in October, because there is no other deal coming."