Geoffrey Cox, the government's top lawyer, is due to give his opinion on Tuesday at 1230 GMT ahead of the vote due around 1900 GMT.
The UK parliament has again rejected Prime Minister Theresa May's Brexit deal after she secured legally binding assurances from Brussels on the controversial Irish "backstop" - but the changes weren't enough to placate lawmakers. Sign-up now and enjoy one (1) week free access!
If lawmakers vote down May's deal, she has promised a vote on Wednesday on whether to leave without a deal and, if they reject that, then a vote on whether to ask the European Union for a limited delay to Brexit.
But as parliament prepares to reassess the deal, the prime minister has little to show for her efforts, prompting warnings of another humiliating loss.
The three-part package of changes effectively aims to resolve a key sticking point for British MPs over the so-called backstop plan to keep open the border between European Union member Ireland and the British province of Northern Ireland.
MPs will vote on the deal at about 7pm. These had equal legal force with the withdrawal agreement, he claimed.
"Discussions are ongoing between ourselves and the EU", May's spokesman told reporters, insisting that Tuesday's vote would take place as planned.
However, he said the situation was unchanged if a deal that supersedes the backstop can not be reached "through no such demonstrable failure of either party". Afterward, hard-core Brexit supporters in May's Conservative Party and the prime minister's allies in Northern Ireland's Democratic Unionist Party both said they could not support the deal.
Theresa May has secured "legally binding" changes to her Brexit deal - just hours before MPs will vote to approve the agreement in the Commons. "There will be no third chance... it is this deal or Brexit might not happen at all", Juncker said, issuing a stark warning to Britain's MPs over the importance of the parliamentary vote in the United Kingdom on Tuesday.
If she loses the vote, she has said MPs will get a vote on Wednesday on whether to leave without a deal and, if they reject that, then a vote on whether to ask for a limited delay to Brexit.
And if that is turned down MPs will have a third vote in three days on whether to extend Article 50 and delay Brexit.
If they vote yes to extend Article 50 then the United Kingdom will ask for a short extension.
What happens after that is anyone's guess, with many MPs, including the Labour party, advocating for a second referendum.
Labour Party leader Jeremy Corbyn urged MPs to vote against the deal when it returns to the Commons, saying May had "recklessly run down the clock" on negotiations.
Currency expert Stafford-Taylor added: "However, if parliament vote against May's deal there will be a fresh vote to consider tomorrow which could further influence to the pound's movement".