Rory Stewart has said rival Conservative leadership contenders who are pledging to ditch the Irish backstop are wasting their time.
The international development secretary told a crowd of around 100 people at Speakers’ Corner in London’s Hyde Park that it was a “siren song” for candidates to promise they would remove it.
The backstop prevents a hard border between Northern Ireland and the Republic in the event of no-deal Brexit.
“Every politician that has spent the last two and a half years saying they can remove the backstop has failed,” Mr Stewart said.
“Europe has no intention of removing the backstop.
“It is a recipe for delay, it is a recipe for uncertainly, and we will still end up in this zombie, purgatory position in the middle of next year if people keep talking this way.
“For heavens sake, please reject the siren song of people saying they would remove the backstop.”
Health secretary Matt Hancock, who is also running for the leadership, has proposed a so-called “time-limited backstop” that would prevent a backstop arrangement indefinitely.
Stewart said the EU would not remove the backstop it negotiated with May, and other Conservative leadership candidates were wasting time by promising to do so.
He elaborated on his proposal to hold a “citizen’s assembly” on Brexit to find a compromise between revoking Article 50 and no-deal positions in the Brexit debate.
If elected PM he would “drive through” the results of the assembly in Parliament and challenge opposition party leaders to do the same, he said.
The assembly would take place without altering the current timeline for the UK leaving the EU in October, and would require no further negotiations in Brussels, he added.