But conflicting signals from diplomats reflected the complexity and sensitivity of the negotiation underway, and the fact that neither party wants to be blamed should the talks collapse. Adding to the sense of uncertainty, European Union diplomats know that Mr. Johnson has no majority in the British Parliament, so that he might struggle to deliver on any deal he strikes.

To have a good prospect of ratifying a new agreement in Parliament, Mr. Johnson needs the support, in particular, of 10 lawmakers from Northern Ireland’s Democratic Unionist Party, who prop up the government but who helped torpedo a previous Brexit agreement negotiated by Mr. Johnson’s predecessor, Theresa May.

Talks with the D.U.P. took place in Downing Street on Wednesday, after which the party’s leader, Arlene Foster, declared that there was more work to do. “‘E.U. sources’ are talking nonsense,” she said in a Twitter post. “Discussions continue. Needs to be a sensible deal which unionists and nationalists can support.”

In a meeting with Conservative lawmakers, Mr. Johnson compared the negotiations to climbing Mount Everest, saying he was on the perilous last approach but that the peak was still “shrouded in mist.”

If there is a deal, and if European Union leaders approve it, Mr. Johnson wants to rush it through Britain’s Parliament on Saturday.