Two crucial votes on Tuesday evening will determine whether the Prime Minister will be able to live up to his "do or die" commitment to take the UK out of the European Union by the October 31 deadline.
French President Emmanuel Macron and EU chief Jean-Claude Juncker had applied pressure on MPs with hints that Mr Johnson's deal is the last and that any further delay may not be granted.
But European Council president Mr Tusk said the latest request for an extension to Article 50 should be treated "in all seriousness" as he consults EU leaders over the requested delay and MPs prepare to debate the Withdrawal Agreement Bill (WAB).
"It is obvious that the result of these consultations will very much depend on what the British Parliament decides, or doesn't decide," he said in a speech at the council on Tuesday.
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The new Brexit deal must also win backing from the European Parliament but its Brexit co-ordinator, Guy Verhofstadt, suggested "all problems faced by EU27 nationals in the UK need to be solved" first.
His demands include no citizens being deported from Britain if they miss the deadline for settled status in order to prevent "another 'Windrush scandal".
Ministers have insisted they are confident they have the numbers to get the WAB through the Commons, despite their defeat in Saturday's special sitting.
However, they face a fierce parliamentary struggle after announcing plans to fast-track it through the lower House in three days, potentially paving the way for the Lords to consider it over the weekend.
They will need MPs to approve a "programme motion" setting out the timetable for its passage through the Commons, setting up a crunch vote on Tuesday evening.
Many MPs are deeply unhappy that there is so little time for detailed scrutiny of such an important Bill, which runs to 110 pages with another 124 pages of explanatory notes.
Rory Stewart, who was among the 21 rebels against no-deal exiled from the Tories, suggested he could back the WAB but not back the programme motion.
• READ MORE: Brexit: England seems to be declaring independence from UK – Angus Robertson"That doesn't mean extending until the end of the year but it does mean we need a few days to do it properly," he told BBC Radio 4's Today programme.
"If we don't do it properly, we are going to undermine the thing from the beginning."
Leader of the House Jacob Rees-Mogg had warned that the programme motion was essential if they were to meet the Halloween deadline.
"People who do not vote for the programme motion will not be voting for Brexit on October 31," he said.
Failure to get the motion through would open up the prospect that Mr Johnson will be forced to accept another lengthy delay to Britain's departure - something he has vowed not to do.
The first vote on Tuesday will be on the Bill's "in principle" second reading, while Labour-backed amendment attempts to secure a customs union and a second referendum could follow.
Shadow chancellor John McDonnell was insisting they would not be "wrecking" amendments but a bid to "secure a decent way forward we could all support".
Despite the opposition of the DUP over arrangements for Northern Ireland, ministers believe they have the support of pro-Leave Labour rebels and former Tory MPs now sitting as independents who would rather leave with Mr Johnson's deal than no deal at all.
If they get through the second reading, the Government will hope it gives the momentum to carry through the programme motion as well.
If that passes, under the proposed timetable, the Bill would then move to the committee stage - which will continue on into Wednesday - when MPs will have the opportunity to put down amendments.
These are expected to include attempts to keep the UK more closely aligned with the EU through a customs union and to stage a second referendum.
Both are bitterly opposed by the Government, raising the possibility that it could pull the Bill altogether if either gets through.