A cross-party trio of pro-EU politicians jokingly branded themselves "the rebels" before holding talks with the European Commission's chief negotiator Michel Barnier.
Former Lib Dem leader Nick Clegg, Tory ex-chancellor Ken Clarke and Labour's Lord Adonis met Mr Barnier in Brussels.
Asked if he was there to stop Brexit, Mr Clegg said: "If only it were that easy."
A European Commission spokesman said Mr Barnier's door was "always open".
But he played down claims of a "shadow negotiation" with the Remain-backers, saying: "There are two negotiators - on the one side the Secretary of State for Exiting the EU, David Davis, and on the European side it's the chief negotiator of the EU, Michel Barnier. Nobody else."
As they arrived, Mr Clarke joked that "we are here to talk about cricket" and did not respond to questions about whether he had permission from the government to attend the meeting.
Mr Clegg said they were there to get a "better understanding" about what was going on in the talks, which have yet to reach a breakthrough on issues including citizens' rights and the UK's financial bill.
But on Twitter, Theresa May's former advisor Nick Timothy said their visit "undermines Britain's negotiating position" and UKIP MEP Gerard Batten told the Daily Express the three politicians had "no right to pretend to represent the British public".
In other Brexit news, the government has published a list of the 58 sectors of the economy on which it has assessed the impact of leaving the EU.
Ministers have refused requests to publish the actual reports, saying this could undermine its negotiating position. The list was published in a response by Brexit Secretary David Davis to a House of Lords committee.