The prime minister summoned members of the Brexit "war cabinet" on Thursday to build support for the terms of the withdrawal agreement before seeking the backing of her full cabinet and the rest of her party.
Downing Street has insisted defeat on the Budget would not amount to a vote of no confidence in the government, but there is no doubt the rhetoric of recent days has ratcheted up the Brexit brinkmanship.
"I'm sure they too will be persuaded the alternatives - of no deal or potentially a Corbyn government - would not be of benefit to them or Northern Ireland", he said.
Instead, the United Kingdom would stay in a customs union, likely to be described as a customs arrangement by the prime minister, on an indefinite basis. "Such a lovely morning" is all she would say to me.
London hopes to resolve the issue with a future trade deal, but agrees there should be a "backstop" arrangement to avoid physical frontier checks until that deal is done.
The EU's chief negotiator, Michel Barnier, told the European commission president and other commissioners in a private meeting on Wednesday that the principles of an agreement on the Irish border are largely accepted by both sides.
"Such a customs union would eliminate an important part of customs checks".
The OBR said if there was no agreement on standards everything would have to be resubmitted for approval: "In a scenario where the United Kingdom and European Union are unable to agree to the continued mutual recognition ('grandfathering') of existing product standards and professional qualifications, all existing goods may need to be re-approved before sale and services trade would be severely restricted by the loss of market access".
Theresa May has been put on notice by her allies in the Democratic Unionist Party to change course on Brexit or risk the collapse of her government.
"Trade from Great Britain into Northern Ireland would be in danger of restriction".
Former Conservative leader and Brexit supporter Iain Duncan Smith warned the prime minister she should "listen very carefully" to the DUP, which he said echoed many Tory concerns.
Instead she proposed Britain as a whole stay temporarily aligned with the EU's customs union until a wider trade deal is agreed.
Barnier told small business leaders that the talks were "continuing intensively this week, day and night, with the aim. of having a deal within reach, if we follow through to the end of this negotiations, on October 17".