Ukip leader Nigel Farage says he's more than a little "nervous" that Conservative positions on Brexit have already softened since the referendum vote to leave the EU.
Worried that the Leave campaign may be "backsliding" over immigration, he also said that failure to deliver sharp cuts in levels of migration from the EU would amount to "selling out" the 17 million voters who backed Brexit.
Leave campaigner - and the man tipped as most likely to succeed David Cameron as Prime Minister - Boris Johnson wrote in the Daily Telegraph that Britain "is part of Europe, and always will be", envisaging "intense and intensifying European co-operation and partnership" even after Brexit. He said the UK would be able to "take back democratic control of immigration policy" and impose a points-based system to suit its own business needs, but expected Britons to be able to work and settle in EU states and have access to the single market.
But Farage urged all Tory leadership candidates to "hold faith" with the referendum result and said he was concerned that Mr Johnson's calls for delay in opening formal negotiations were a "smokescreen" for an intention to keep Britain within the EU's freedom of movement system, which allows citizens to live and work elsewhere in the continent.
The Ukip leader told Channel 4 News: "I don't care who the next Conservative leader is, as long as it's someone who will hold faith with that referendum result. I think over the next week or two we will see the names coming forward and we will see precisely what they stand for.
"I think that Boris has a lot going for him in terms of public appeal, but perhaps some of the things he's been saying over the last 48 hours indicate there might be a slight softening of his position.
"(He says) that there's no rush, which can be fine but does he really want to deal with this free movement issue? I don't yet know the answer to that. I'm worried that that's being used as a smokescreen to do a different kind of deal with the EU that would keep us within free movement.
"I'm pretty cautious about some of the motives of those who are saying we should take our time. I'm nervous. I'm more nervous than I was on Friday morning. I'm beginning to hear noises, I'm beginning to detect there may be some backsliding and I don't find that acceptable."
Asked whether he was concerned that the Leave campaign may have fuelled a spate of racist incidents reported over the weekend, Mr Farage said: "There was nastiness on both sides, and I think what we've got, post the referendum, of very, very heightened feelings, and some bad things being said on both sides.
"I've never, ever, ever encouraged or condoned behaviour like that, and I never, ever would."