The European Union has denied holding parallel Brexit negotiations after a trio of Remain campaigners headed to Brussels.
Former deputy prime minister Nick Clegg, ex-Conservative chancellor Ken Clarke and Labour peer Lord Adonis met with EU lead negotiator Michel Barnier and other figures on Monday.
EU officials described the talks as part of Mr Barnier’s open-door approach, but stressed there is only one set of negotiations between Britain and the bloc.
Asked whether he had travelled to the European Commission’s offices in the Belgian capital to stop Brexit, former MP Mr Clegg told the BBC: “If only it were that easy! We’re here just to get a better understanding of what’s going on in the talks.”
Responding to suggestions the meeting would lead to claims the EU has set up a “shadow negotiation”, European Commission spokesman Alexander Winterstein said: “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.
He added: “All these meetings were requested by the people who are coming to see Mr Barnier. His door is always open.”
The meeting infuriated Brexit campaigners, with the trio all opposed to Britain’s exit from the EU.
Mr Clegg has recently published a book titled How To Stop Brexit, while Mr Clarke was the only Tory MP to vote against the triggering of Article 50 earlier this year.
Meanwhile, former Cabinet minister Lord Adonis has compared Brexit to Britain’s appeasement of the Nazis in the 1930s.
Former UKIP leader Nigel Farage said: “It’s like an Unholy Trinity going down and desperate to undermine the democratic Brexit vote.
“Especially so, as Clegg has no electoral mandate.
“That Michel Barnier is even meeting them shows those in Brussels cannot be trusted in these negotiations at all.”
Mr Farage later wrote to Mr Barnier to demand his own meeting, claiming “to speak for the 17.4million who did not vote for a transition deal or any further delays” over Brexit.
Brexit supporters previously reacted with anger when former prime minister Tony Blair, who has raised the prospect of a second EU referendum to overturn the Leave vote, met with European Commission president Jean-Claude Juncker in the summer.
Meanwhile, top government ministers will discuss the next stage of the Brexit process at a Cabinet meeting on Tuesday, Downing Street revealed.
This will include discussions on the Prime Minister’s hopes for a trade agreement, as well as contingency preparations in the event no deal is agreed between the UK and EU.
The European Commission said discussions were still ongoing over the date of the next round of Brexit negotiations.