Jeremy Corbyn will throw himself into the middle of the negotiations with the EU by holding talks with Michel Barnier in Brussels just hours before Theresa May seeks to persuade leaders at a summit to accept a short delay to Brexit.

The EU’s chief negotiator disclosed to ministers for the 27 member states on Tuesday evening that the Labour leader had sought the meeting.

Corbyn will see Barnier in the EU’s Berlaymont headquarters on Thursday morning. A few hours later in the Belgian capital, Theresa May will appeal to the EU’s heads of state and government to accept her expected request for a three-month delay to Brexit.

Corbyn is also attending a meeting of fellow socialist opposition leaders and prime ministers, such as Spain’s, Pedro Sanchez, which is taking place in Brussels in the morning.

The appearance of Corbyn in Brussels at what Barnier told reporters on Tuesday evening was a “very, very sensitive” time will be cause of concern in Downing Street.

There will be fears that EU leaders will receive mixed messages as they mull over the prime minister’s formal proposal to prolong the article 50 negotiating period. A delay will require unanimous approval by the EU’s leaders.

The meeting will give Corbyn an opportunity to discuss again the outline of his vision for Brexit, and update Barnier on the cross-party talks.

The EU has been pushing May to shift her red lines and sign up to permanent customs union in order to garner Labour support for her deal.

Barnier will is also likely to be interested in the suggestion that Labour could abstain on another vote on the deal to allow it pass on the basis that there would then be a confirmatory referendum.

The SNP, Lib Dems, Plaid Cymru and Greens had urged Corbyn during a cross-party meeting on Tuesday to throw his weight behind a second Brexit referendum.

Speaking on Tuesday evening, Barnier had said that if May could not get her current deal past parliament, the EU would be open to a long extension but only if Downing Street changed tack or there was a “new event” such as a general election or second referendum.

“I recall this political declaration, which sets out the framework of our future relations, could be made more ambitious in the coming days if a majority in the House of Commons so wishes,” Barnier said.

“If not, what would be the purpose and outcome of an extension? And how can we ensure that at the and of a possible extension we are not back at the same situation as we are today? In any case, the EU council will need to access what is the best interests of the EU.”

He added: “My feeling is that a longer extension needs to be linked to something. There needs to be a new event, a new political process and obviously I cannot attempt to preempt such a process.”

The EU is not legally allowed to set conditions on an extension, but understandings are being sought. May’s de facto deputy, David Lidington, was engaged on Tuesday in intensive preparatory talks in Brussels on a way forward.