Boris Johnson is facing a fresh Brexit headache after opposition MPs claimed they had enough support to force the prime minister to negotiate a customs union with the EU.

Labour said there was a “very good chance” of a customs union motion being passed by the Commons later this week, as the DUP appeared to indicate that it could support the plan.

In a move that could scupper the government’s pledge to deliver Brexit by 31 October, the Northern Irish party hinted that it might join forces with Labour and other opposition parties to vote for a closer economic relationship with the EU after Brexit.

The move would likely force Mr Johnson to ditch plans to enact his deal in time for the Halloween deadline and instead accept the inevitability of a further delay to Brexit. He would then be expected to turn his efforts to trying to trigger a general election.

Labour is expected to table a customs union amendment later this week when MPs vote on the Withdrawal Agreement Bill, which would implement Mr Johnson’s Brexit deal in law.

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The motion is likely to be backed by almost all opposition party MPs and most of the 21 former Tories expelled by Boris Johnson last month.

Amid speculation that the DUP could also support it, the Daily Telegraph quoted a senior party sources as saying: “Clearly that would be one way we could look at addressing our concerns. There will be some very grown-up conversations over the next 48 hours and we will be looking at all the options we have available.”

The source said the party would launch “parliamentary guerilla warfare” against Mr Johnson in a bid to defeat his deal, which it opposes because the agreement would create a new customs and regulatory border between Northern Ireland and the rest of the UK.

The threat came amid growing confidence among opposition parties that they have the numbers needed to pass a customs union amendment.

Nick Thomas Symonds, Labour’s shadow solicitor general, told the BBC’s Westminster Hour: “A customs union is going to be one of the amendments that comes forward and I think that is something that is going to have a very good chance of getting a majority.”

If passed, a customs union amendment would cause major problems for Mr Johnson because it would likely force him to either agree to the demand or pull the entire bill, whic is necessary to implement his deal.

Proposals for a customs union came within a handful of votes of being approved when MPs held “indicative votes” on a range of possible Brexit outcomes earlier in the year.

The latest move could convince some Labour MPs who were contemplating backing Mr Johnson’s deal to demand that he accept plans for a customs union.

Gloria de Piero, one Labour MP who was believed to be considering voting for the deal, tweeted: “A customs union amendment is what I’ll be working for. It is the option that was three votes short of passing in Parliament. I am convinced we can get majority support for it. It would enable us to stay true to the Labour manifesto every Labour MP was elected on.”

Liz Truss, the international trade secretary, claimed that a customs union with the EU would “take us back to square one and render the UK unable to strike our own bilateral trade agreements around the world”.

Writing in the Telegraph, she said: “This week, we have another crucial opportunity to end this uncertainty and take the UK out of the EU with deal. And think of where we could be in December 2020 if we do? Free trade deals negotiated with the EU and other partners that ensure our future as a high skilled, innovative, free enterprise economy, excelling in tech, data and services and selling our high-quality food around the world.