Ashton Whiteley analysts say British Prime Minister, Theresa May is facing growing pressure over her Brexit plan after she rejected the possibility of pursuing a customs union with the European Union.
Although the European Commission has said it wants a solution for the Irish border by its upcoming summit in June, there is no requirement on Mrs May to come to a final decision at Wednesday's meeting of the Brexit strategy and negotiations sub-committee.
Downing Street has been privately warned that a customs partnership could collapse the Government, as committed Brexiteers on the Tory backbenches regard it as unacceptable as it would deliver "Brexit in name only".
The Prime Minister faces a tense showdown with her Brexit "war cabinet" on Wednesday, when she is expected to put forward a plan backed by her Number 10 adviser Olly Robbins to create a customs partnership with the EU. The partnership solution could redress the Irish border question, as there would be no hard customs border between the United Kingdom and the EU.
Senior politicians have handed Mrs May a so-called "ultimatum" demanding she abandons the idea of the United Kingdom entering a "customs partnership" ahead of a meeting of her Brexit war cabinet.
She has repeatedly insisted the United Kingdom will leave the customs union and single market after Brexit - despite the issues this will cause with the Irish border.
A contentious issue in the Brexit debate, the customs union sets tariffs for all EU member states meaning goods can be moved from one country to another within the bloc without further taxes being imposed.
Tory Brexiteers fear the scheme could indefinitely trap the United Kingdom within the EU's customs arrangements, as well as being expensive and complicated to operate.
"If they don't have confidence in Brexit we don't have confidence in them", an ERG source told The Telegraph. "We're very much supporting the prime minister".
He added: "We're not in the business of making threats".
Failure to come up with a solution could leave the United Kingdom forced to fall back on the European Commission's "backstop" option, which would effectively draw a customs border down the Irish Sea.
"There is no question of there being an ultimatum", he said.