Diplomats and officials said Wednesday's discussions were a sign of determination to cope with a "hard Brexit" if Britain refuses to accept European Union terms, but views were mixed on whether it signalled a greater degree of urgency or fears of a no-deal.
This must include membership of Europe's customs union, no hard border in Ireland, and the protection of workers' rights and environment and consumer standards, he said.
Watching the British political debate - including talk of a snap election or even a new Brexit referendum - another diplomat said there was growing unease in Brussels that May would be unable to get unpalatable compromises over Northern Ireland's borders and United Kingdom trade access to the European Union through parliament.
Corbyn will make a speech to close the Labour Party conference today.
Theresa May has rejected Labour's demands for an early general election, saying a snap vote "would not be in the national interest".
And MP Gareth Snell said Sir Keir's "unilateral declaration" about the possibility of a Remain option was "disappointing".
The motion's wording was agreed on Monday after five hours of intense talks and it disappointed many party members, who had hoped for a firm commitment to a second vote on a final Brexit deal, including the option to remain in the EU. "Unless we offer radical solutions, others will fill the gap with the politics of blame and division".
"Ten years ago this month, the whole edifice of greed-is-good, deregulated financial capitalism, lauded for a generation as the only way to run a modern economy, came crashing to earth, with devastating consequences".
In his speech, the Labour leader will also outline plans for a "green jobs revolution" to create 400,000 skilled posts.
British Labour Party leader Jeremy Corbyn has promised to recognize Palestinian state as soon as his party takes office after winning the next elections.
"Inequality is not just a matter of incomes".
Mr Corbyn sought to draw a line under the anti-Semitism scandals which have dogged Labour over the summer, promising to "eradicate anti-Semitism, both from our party and wider society" and telling the Jewish community: "We are your ally".
He declared Labour represents "the new political mainstream" and offers voters "an alternative to the politics of austerity, of social division and of global conflict".
Labour leader Jeremy Corbyn has so far played a canny game by allowing the debate to tie the government in knots while remaining ambiguous, but the time is nearing to reveal his hand. And we are ready to deliver on it.
The British Chambers of Commerce said there were some "bright spots" but "Jeremy Corbyn missed a golden opportunity to extend an olive branch to British business".
But no one can respect the conduct of the government since that vote took place.