Democratic Unionist MPs who prop up Theresa May’s government are preparing to vote against her Budget if the Prime Minister breaks their Brexit red lines.
The radical move is understood to be one of the options being considered by the DUP if attempts to nail down a deal with Brussels include any proposals that would leave Northern Ireland being treated differently to the rest of the UK.
Losing the party’s support in the House of Commons would mean possible defeat on the budget and a no-confidence vote.
The development came as former foreign secretary Boris Johnson warned that a “backstop” arrangement for Northern Ireland being negotiated by the Government would leave the UK “a permanent EU colony”.
In a series of tweets, Mr Johnson said that the deal would keep the UK in the customs union and Northern Ireland in the single market, and would mean increased checks on goods travelling between the province and the British mainland.
Repeating his call for Mrs May to ditch the plan agreed at her country residence in July, he said: “In the referendum both sides said Leave meant leaving the customs union and single market. Yet this backstop inevitably means Chequers, staying in both, no say in either, and no right to escape.”
DUP leader Arlene Foster insisted on Tuesday that her party would not accept customs or regulatory checks on goods travelling in either direction between Northern Ireland and Great Britain after meeting EU chief negotiator Michel Barnier in Brussels.
A DUP spokesman said: “The Government is well aware of our position on this issue. Our position hasn’t changed and we don’t expect the Government will change its position.”
Mr Barnier told a business audience in Brussels that agreement on a withdrawal deal was “within reach” at the crunch October summit of the European Council next week, with “80-85 per cent” of the accord now finalised.
But he made clear that the EU envisages a deal involving new customs and regulatory checks on goods travelling from mainland Britain to Northern Ireland, including health and sanitary inspections for all animals and animal products arriving in ports like Larne and Belfast from the rest of the UK.
Mr Barnier said checks would be carried out “in the least intrusive way possible”, adding: “I understand why such procedures are politically sensitive but ... Brexit was not our choice, it is the choice of the UK.”
The DUP has ten MPs, although Ian Paisley will not be able to vote until 20 November after being suspended for failing to declare two family holidays paid for by the Sri Lankan government.