Theresa May is under pressure over evidence she is planning a “long” delay to Brexit if MPs still refuse to back her deal, after her chief negotiator was overheard in a Brussels bar.

Anti-EU Tories reacted angrily after Olly Robbins appeared to reveal the strategy – contradicting the prime minister’s insistence that she is ready to crash out of the EU without a deal if necessary.

“Extension [of Article 50] is possible but if they don't vote for the deal then the extension is a long one,” he was heard saying, by an ITV News journalist.

“The issue is whether Brussels is clear on the terms of extension. In the end they will probably just give us an extension.”

The comments also appeared to confirm that Ms May, as a cabinet minister hinted yesterday, plans to delay the critical decision until the eve of the scheduled departure day on 29 March.

Steve Baker, the deputy leader of the hardline European Research Group of Tory MPs said Mr Robbins should be “appalled by this story”. Downing Street declined to comment.

The row came as the ERG also warned it could vote against the government’s standstill motion on Brexit in a Commons debate on Thursday – which would plunge Ms May into a fresh crisis.

Police separate clashing Brexit protesters outside Parliament Show all 11 1 /11 Police separate clashing Brexit protesters outside Parliament Police separate clashing Brexit protesters outside Parliament Anti-Brexit protester Steve Bray (left) and a pro-Brexit protester argue as they demonstrate outside the Houses of Parliament Getty Images Police separate clashing Brexit protesters outside Parliament A pro-Brexit protester argues Getty Images Police separate clashing Brexit protesters outside Parliament Police surround the pro-Brexit protester after he confronted Steve Bray, a pro-European protester Getty Images Police separate clashing Brexit protesters outside Parliament A leave supporter is spoken to by a police officer as he argues with a remain supporter, Steve Bray, outside Parliament PA Police separate clashing Brexit protesters outside Parliament MPs in Parliament are to vote on Theresa May's Brexit deal next week after her December vote was called off in the face of a major defeat Getty Images Police separate clashing Brexit protesters outside Parliament A leave supporter is spoken to by a police officer PA Police separate clashing Brexit protesters outside Parliament Police look on as anti-Brexit protesters demonstrate outside the Houses of Parliament. Getty Images Police separate clashing Brexit protesters outside Parliament Police hold back a leave supporter PA Police separate clashing Brexit protesters outside Parliament A police officer speaks with anti-Brexit protester Steve Bray Getty Images Police separate clashing Brexit protesters outside Parliament Police surround a pro-Brexit protester after he confronted a pro-European protester Getty Images Police separate clashing Brexit protesters outside Parliament Conservative MP David Davies, wearing a gopro camera, speaks to anti-Brexit protester Steve Bray Getty Images

The motion backs “the approach to leaving the EU expressed by this House on 29 January”, which – as well as calling for the Irish backstop to be replaced – also saw a separate vote ruling out a no-deal Brexit.

An ERG source told the Brexit Central website that the motion passing would “destroy our leverage in the critical negotiations with the EU”, adding: “This is utterly chaotic, bordering on farce.”

In the overheard conversation, Mr Robbins also sparked a second controversy by suggesting the backstop – designed to avoid a hard border in Ireland – was conceived as a “bridge” to the future relationship.

The comment was seen as revealing it is meant as the basis of a permanent relationship, which could keep the UK in an EU customs union, something Ms May denies.

An ERG spokesman added: “Who's in charge of this? Either the prime minister is and this is what she wants, or she isn't and is just doing what the civil service tells her.”

In his report, the ITV News reporter admitted he could not hear every word spoken by Mr Robbins and that the bar was too noisy to make a recording.

Asked whether the comments reflected government policy, Brexit Secretary Stephen Barclay told BBC Radio 4's Today programme: “No.”

He said: “The prime minister has been very clear that we are committed to leaving on 29 March.”