Cross-party Brexit talks between the government and backbench MPs have resumed despite Theresa May’s insistence that her first priority is to reach out to hardline Tory Brexiters and the Democratic Unionist party over the issue of the Northern Ireland backstop.

The anti-Brexit campaigners Chuka Umunna and Anna Soubry were among a group of MPs who arrived at the Cabinet Office for a pre-arranged meeting with May’s de facto deputy, David Lidington, and the Downing Street chief of staff, Gavin Barwell.

Labour’s Umunna and Soubry of the Tories are co-chairs of a cross-party group that has taken the lead on tabling a slew of amendments to Brexit legislation over the past year. Both are vocal advocates of a “people’s vote” – a second referendum – as a way of resolving the impasse in parliament.

Speaking to reporters before the meeting, Soubry said: “This now has to get into parliament and we have to take control of things. And the most important point is this: we are reaching the point where parliamentarians have to decide whether or not they are simply going to vote for something again which has not been successful, or do the right thing, which is to take this back to the British people.”

The prime minister is expected to tell MPs in a statement later on Monday that she will make another attempt to renegotiate the Irish backstop with the EU27, rather than try to find a cross-party compromise. May intends to focus instead on winning over her own backbenchers and the DUP’s 10 MPs.

Whitehall sources suggest Barwell is more warmly disposed towards the idea of seeking a cross-party solution, perhaps by embracing the idea of a customs union, but May appears to be set against it.

The prime minister’s withdrawal agreement was defeated by a crushing majority of 230 in the “meaningful vote” last week. Afterwards said she would hold a series of meetings with “senior parliamentarians” to try to find a deal that could win the backing of MPs.

Umunna’s attendance at the meeting was in defiance of a request from the Labour leader, Jeremy Corbyn, for MPs not to engage in further talks with the government. His Labour colleagues Chris Leslie, Luciana Berger and Gavin Shuker were also among the group.

Umunna told reporters: “The Labour MPs who are part of this delegation – obviously we were instructed not to meet with the government, but you’ve got to put the national interest first, the party politics to one side. We were not going to pass up the opportunity to press our case for a people’s vote today.”

He said there had been speculation that there would be no more cross-party talks, adding: “The fact that we’re here illustrates that the government is not in control of this process.”

Corbyn refused an invitation to meet May last week, saying he would not do so until she took the possibility of a no-deal Brexit off the table. Downing Street sources say the prime minister concluded she could not rely on Labour to deliver a majority.

Some pro-Brexit Tories appear to have softened their language around May’s deal in recent days. The European Research Group’s Jacob Rees-Mogg wrote in the Mail on Sunday: “If I had to choose between no deal and Mrs May’s original accord, I would have no hesitation of opting for a no-deal Brexit but even Mrs May’s deal would be better than not leaving at all.”