DUBLIN (Reuters) - Details of the draft withdrawal treaty between Britain and the EU reported on Tuesday “fundamentally breach” a deal reached last year to move Brexit talks forward, a member of the Northern Ireland party that props up Britain’s minority government said.

Democratic Unionist Party (DUP) Leader Arlene Foster, alongside Deputy Leader Nigel Dodds and Chief Whip Jeffrey Donaldson, leave 10 Downing Street, in central London, Britain June 26, 2017. REUTERS/Stefan Wermuth

The draft to be agreed by the executive European Commission on Wednesday will cross many British red lines, officials say, and include controversial elements such as a clause preventing “regulatory divergence” across the Irish border.

The clause will put into legal effect an agreement reached by negotiators in December where Britain pledged that Northern Ireland, which will be the UK’s only land frontier with the EU after its departure, will, in a worst case scenario, stay aligned with the rules of the EU’s single market and customs union.

However in a sign of potential difficulty for British Prime Minister Theresa May, a senior member of the Democratic Unionist Party (DUP) reacted angrily to a report by Irish broadcaster RTE that a concurrent pledge that there will be no barriers between Northern Ireland and the rest of the UK will not be in the text.

“This fundamentally breaches the understanding reached in December and would undermine the constitutional status of NI,” Jeffrey Donaldson, one of 10 DUP lawmakers whose support May relies on in parliament, said on Twitter.

“If the EU or Dublin believes the UK Government will be signing up to a border in the Irish Sea, they are deluded.”

The draft text will also state that, under the regulatory alignment backstop option, joint EU-UK customs teams will be required to apply checks on goods coming from the UK into the new regulatory space on the island of Ireland, RTE reported, quoting an EU source.

British officials are also uneasy about EU negotiators writing into the draft that the British province will avoid diverging from EU rules.

Opposition from the DUP to the initial wording of December’s agreement almost scuppered the breakthrough on the border and Irish Prime Minister Leo Varadkar anticipated earlier on Tuesday that there could be similar problems ahead.

“I’m satisfied that it (the draft withdrawal treaty) puts down in law how we can avoid a hard border between Northern Ireland and Ireland,” Varadkar told parliament.

“But I should say the withdrawal agreement being published tomorrow is a draft text and we can’t automatically assume that will be acceptable to the United Kingdom or all the parties in Northern Ireland so we could have an interesting few weeks ahead, as we had back in December.”