British Attorney General Geoffrey Cox said on Tuesday that the newly secured legally binding changes to the Brexit deal will "reduce the risk" of his country being indefinitely and involuntarily held in the Irish backstop.
May announced three documents - a joint instrument, a joint statement and a unilateral declaration - which she said were aimed at addressing the Irish backstop, the most contentious part of the divorce deal she agreed with the European Union in November.
"The choice is clear: it is this deal, or Brexit may not happen at all", Juncker said.
With just a little over two weeks until the deadline of March 29, May has stated that she will hold a vote Wednesday to determine whether or not the United Kingdom should pursue a no-deal Brexit if Parliament votes down the measure on Tuesday.
May said the changes meant the Irish backstop - the insurance policy created to avoid a hard border between the Irish Republic and the United Kingdom province of Northern Ireland - could not "become permanent", the BBC reported.
Shadow Brexit secretary Sir Keir Starmer said Cox had confirmed that "no significant changes" had been secured to the Withdrawal Agreement and the government's strategy was "in tatters".
It's reported the DUP will be voting against the deal and not abstaining.
The so-called European Research Group (ERG) of "hard Brexiteer" Tories will meet later on Tuesday to decide whether its several dozen members should back the new deal.
"If we vote for the improved deal we will both end the current uncertainty and deliver Brexit", he said.
One eurosceptic lawmaker told Reuters he had seen nothing to change his opposition to the deal so far but that lawmakers were awaiting a legal analysis.
Mr Cox looked at the changes that Mrs May had agreed, but concluded that the legal risk had not changed.
The Democratic Unionist Party, whose support May relies on in the Commons, said it would be "scrutinising the text line by line" before deciding whether to back the deal. However, even if the Article 50 - the notification that Britain will leave the European Union - extension is granted, Juncker said last night that "it's this deal or Brexit might not happen at all".
His legal opinion sent Sterling sharply lower and dented risk appetite more generally as the chances rose that May will lose the vote on her deal in the UK Parliament that is due to take place at 1900 GMT.
"It's likely the pound will react in light of the aftermath of tonight's vote - but in which way depends on the outcome of the vote".
Some British lawmakers had warned their Brexit-backing colleagues that rejecting the deal could lead to Britain's departure being postponed indefinitely, because a delay would give momentum to opponents of withdrawal.
Does Theresa May have the votes in the House of Commons?