LONDON (Reuters) - The Northern Irish party which props up British Prime Minister Theresa May will back an amendment proposed by rebel Brexiteer lawmakers that will effectively make the European Union’s backstop proposal illegal, a Telegraph newspaper reporter said.

Brexit talks have stalled over a disagreement on the so-called Northern Irish “backstop”, an insurance policy to ensure there will be no return to a hard border on the island of Ireland if a future trading relationship is not agreed in time.

May has rejected the EU’s proposal - for Northern Ireland to remain in the bloc’s customs union - as it would potentially create barriers to trade with the rest of Britain.

Prominent Brexiteer Steve Baker, a former junior Brexit minister, will try to block the EU’s backstop plan on Wednesday by attaching amendments to legislation passing through parliament.

Northern Ireland has been without a fully functioning executive since January 2017, and Baker’s amendment would prevent officials acting in the absence of a devolved government to implement any trade or regulatory barriers between Northern Ireland and mainland Britain.

“DUP will back amendment being tabled by Tory Eurosceptics on Wednesday that will effectively make the EU’s NI backstop illegal,” Steven Swinford, the Telegraph’s deputy political editor, said on Twitter.

“Yet another headache for No. 10, with 40 Tory MP already poised to support it.”

The Democratic Unionist Party, whose 10 lawmakers provide May with a slim working majority in parliament, say the EU’s backstop proposal would tear Northern Ireland from the rest of Britain. The DUP had no immediate comment on the report.

Only seven Conservative lawmakers have to rebel for May to lose a vote in parliament, if all opposition party lawmakers vote against the government, so if 40 Conservatives backed the amendment the DUP’s decision would not sway the outcome.

But the scale of votes against the government could provide an indication of how many may ultimately refuse to back any final Brexit deal with the EU when it is voted on in parliament.

The Times reported that the opposition Labour Party was likely to abstain on the amendment, however, meaning the government would still win the vote. A spokesman for the Labour Party said it would make a decision later on Monday.

May’s spokesman said the government would look at the amendments “in the usual way”.

“It’s worth pointing out the legislation’s purpose: it is necessary to provide the Northern Ireland civil service with the certainty and clarity they need to continue to deliver public services,” he told reporters.