Boris Johnson’s attempts to pass his Brexit deal are still on a knife-edge, despite senior ministers claiming he has the numbers to get it through, according to Guardian analysis of likely voting intentions.

Johnson appears to have got all of his 287 voting MPs on board with his plan, including his brother, Jo Johnson, who backed a second referendum but voted with the government on Saturday.

He also has about 20 former Conservatives who left the party or had the whip suspended, plus likely votes from about eight Labour MPs.

Another three independent MPs – former Labour ministers Ian Austin and Frank Field, plus former Liberal Democrat Stephen Lloyd – are all expected to back him.

However, that would still leave him one short of a majority for his deal, if abstentions follow the same pattern as previous votes.

The voting intentions of several independents and a handful of undecided Labour MPs will therefore be crucial. John Woodcock and Ivan Lewis, two former Labour MPs, gave the government hope that they could back a deal by voting with it to avoid an extension to article 50 on Saturday.

But Sylvia Hermon, the independent unionist MP, demanded a clear “guarantee that there is nothing in this deal, this new Brexit deal, which undermines or weakens the constitutional status of Northern Ireland,” in the House of Commons on Saturday. She had previously voted for May’s deal, but her support for Johnson’s deal appears to be in doubt.

No 10 has been working on Labour MPs and independents for several days in an effort to get them to back a deal. The withdrawal bill due to be published this week could contain concessions on giving parliament a say at future points in the Brexit process. Victoria Prentis, a Conservative MP, has been a key figure in the government’s attempts to persuade Labour MPs to get behind Johnson’s deal.

The support of more Labour MPs could depend on the form that the vote takes. Lucy Powell, the Manchester Central MP who was one of a group of 19 MPs in the party who urged the EU to work to find a deal with Johnson before last week’s resolution, said she could possibly back an agreement if it was decided through legislation rather than a straight yes or no vote.

She told Sky News: “I probably wouldn’t have voted for it if we had had a straight up and down vote on it over the weekend but I do think now that we’ve got to move forward. You do have to resolve this issue, you have to resolve Brexit, even if you are somebody who is advocating a second referendum you still need to say what’s going to be on the ballot paper.

“I think it probably now is boiling down to a Brexit of this deal versus a general election and I would really worry about a general election when we haven’t actually left the EU. I worry about what that would do to the Labour party and the Labour communities that we represent.”

There are 13 more Labour MPs who signed the letter saying they were in favour of leaving the EU with a deal, but they all appear to be still undecided. One of them, Emma Lewell-Buck, released a statement saying it had been a “tough call” as she did not want “endless” delay but she had voted for the Letwin amendment forcing an article 50 extension to be requested so that MPs could scrutinise the Brexit legislation.

Despite the uncertainty around the vote, Dominic Raab, the foreign secretary, told the BBC’s Andrew Marr Show: “We seem to have the numbers in the House of Commons. A lot of people say: ‘Get this done and move on.’”

Almost a week ago, before an agreement with Brussels had been finalised, Jacob Rees-Mogg, the leader of the House of Commons, claimed the votes were there for Johnson’s deal.

No 10 appears to believe that there are more Labour MPs who will end up backing the prime minister’s deal, despite them having voted against May’s softer deal three times previously and coming under serious pressure from colleagues not to do so.

Play Video 1:41 Michael Gove insists UK will leave EU by end of October – video

Michael Gove, the chancellor of the Duchy of Lancaster, said on Sunday he still believed the UK would leave on 31 October with a deal, meaning MPs will have had to vote in favour of one by that date.

“That’s our determined policy. We know that the European Union wants us to leave, we know that we have a deal that allows us to leave and listening to Labour MPs like Lucy [Powell] who are very honourably wanting to make sure that the referendum mandate is fulfilled, there is now an opportunity with a good deal,” he said.