BORIS Johnson has begun a grand charm offensive to shore up Tory Brexiteer support for his “great new deal” on Brexit while seeking to win over Labour MPs representing Leave constituencies.

The votes from both groups look set to be crucial if the Prime Minister has any chance of winning tomorrow’s “Super Saturday” vote at Westminster, which looks certain to be on a knife edge.

Labour’s John McDonnell this morning warned any colleague, thinking of rebelling and backing the Government, that there would be “consequences” if they did so; but he stopped short of saying the whip would be removed.

The SNP sought to put its own pressure on any Labour MP considering joining the Conservatives in the Commons Lobbies, saying they would “never be forgiven in Scotland” if they facilitated a Tory Brexit.

READ MORE: Live updates: Scottish court to hear bid to ban Commons vote on PM Boris Johnson's Brexit deal

The DUP, which has rejected Mr Johnson’s deal saying it would “siphon off” Northern Ireland from the rest of the UK and undermine the integrity of the Union, reaffirmed its opposition and said they would push for a general election to help secure "more robust" divorce terms.

But it too sought to put pressure on MPs, telling "Conservative and Unionist MPs” that they must “take a stand for the Union and join us in rejecting this deal”.

"Internal & burdensome trade barriers will be erected within the UK without parallel consent from both Unionists & Nationalists. This is not Brexit," tweeted Sammy Wilson, the DUP’s Brexit spokesman.

The key groups are: the members of the Conservative Brexiteers’ European Research Group, said to number as many as 70 MPs and who include the 28 so-called Spartans, who rejected Theresa May’s deal, those 21 Tories who were expelled; the raft of other independent MPs and Labour MPs, who represent seats which voted Leave. Some 19 of them led by Caroline Flint wrote to the European Commission earlier this month saying they would back a deal if one was agreed with the UK.

Mr Johnson, who will hold a special pre-vote Cabinet meeting at 4pm, is expected throughout the day to have a series of private meetings and phonecalls to try to convince parliamentary colleagues of the merits of the UK-EU’s second plan for Brexit after skipping the second day of the European Council meeting in Brussels.

After he clinched the deal with the EU27 the PM urged MPs to "come together and get this thing done".

But Mr Wilson made clear his party would continue to hold firm and would vote against the Brexit deal on offer.

“I can give you absolute assurance we will not be voting for this deal when it comes before the Commons tomorrow," he told BBC Radio’s Today programme.

The MP for East Antrim suggested a trip to the polls could help Mr Johnson secure "more robust" terms with the European Union, arguing that leaders in Brussels saw him as "vulnerable" in Parliament without a majority.

"I believe, with a big majority, he can be more robust in his negotiations," declared Mr Wilson.

Dominic Raab stressed the Government would continue to engage with the DUP to "give them the reassurances we can".

But the Foreign Secretary defended moves to allow Northern Ireland to stay aligned with EU regulations and customs rules via a "simple majority" in Stormont, rather than need to gain the consent of both communities.

He claimed the UK and EU had settled upon a "reasonable approach" to solving the "undemocratic" nature of the backstop envisioned in Mrs May's rejected deal.

READ MORE: How a No Deal Brexit will affect me: Scottish councils' fears for EU exit revealed

"It seems to me a simple majority of the Northern Ireland Assembly to take a decision to remain aligned to those limited rules necessary in order to guarantee no infrastructure at the border, to guarantee seamless access to the EU single market, is a reasonable approach," argued Mr Raab.

But Green Party MP Caroline Lucas tweeted: "The more you look into PM deal, the worse it is It trashes environment, dumps workers' rights & stokes tension in NI. And when transition ends in Dec 2020, likely no trade agreement agreed & hard right will get their dream of crash out No Deal Brexit I'll be voting against."

Chuka Ummuna for the Liberal Democrats denounced the PM’s deal as a "gangplank into thin air," which would “do over the British people".

He pointed out that even if Mr Johnson got his deal through on Saturday, it would not be the end of the matter.

"We are going to have years more of negotiations and we will be on this cliff-edge again at the end of next year because there will not be a trade deal agreement, there's not enough time to negotiate that with the EU by the end of next year," added the London MP.

While the parliamentary arithmetic looks incredibly tight and tomorrow’s key vote could be won or lost by just a handful of votes either way, Mr Johnson insisted he was "very confident" that he would win the day,

He has been boosted in his view by former Tories he sacked following their rebellion over a no-deal Brexit, who have made clear they are considering voting for his plan.

Sir Nicholas Soames, the former Defence Minister, said he would vote in favour of the deal and that his other 20 colleagues who had the whip removed would "by and large vote for it".

Attention has also turned to the Labour Party's own psychodrama and its troubles over the looming meaningful vote, with the focus on what MPs in Leave-voting seats will opt to do.

It is suggested that between 10 and 15 of them will be prepared to back the deal to avoid a no-deal scenario.

Ronnie Campbell, who represents Blyth Valley in Northumberland, confirmed he would back the divorce terms and his Labour colleague Ruth Smeeth MP, who represents Stoke-on-Trent North, a constituency that voted by more than 70 per cent to Leave, said it was her "intention" to vote for a deal as long as it did not erode rights in Britain.

READ MORE: How a No Deal Brexit will affect me: Scottish councils' fears for EU exit revealed

Mrs May conspicuously sought to allay the fears of some Labour MPs over workers’ rights and environmental protection but in the new deal these have been shifted from the legally-binding Withdrawal Agreement into the aspirational Political Declaration. It is possible that before the vote tomorrow the Government will move to underline its commitment on these areas to help ease Labour MPs over the line.

Meanwhile, Nicola Sturgeon has suggested it would suit Labour for enough rebels to cross the threshold and push Mr Johnson's deal over the line.

"Hope I'm wrong,” the First Minister tweeted, “but I have a real suspicion that Labour would be quite happy to see this deal go through. They will officially oppose but give nod to 'rebels' to ensure numbers there to pass."

Her colleague Kirsty Blackman, the party’s deputy leader at Westminster, said: “If Labour MPs vote for Boris Johnson’s appalling Tory deal and facilitate Brexit, they will never be forgiven in Scotland.”

The Aberdeen MP claimed Scotland had been “completely ignored by Westminster” throughout the Brexit process and was now being “singled out for a raw deal as the only part of the UK that will not get what it voted for and have no say at all”.

She said: “Jeremy Corbyn must guarantee that all Labour MPs will vote with the SNP against this deal, which would be devastating for Scotland; ripping us out of the EU, single market and customs union against our will, terminating our freedom of movement rights, and threatening jobs, living standards, public services and the economy.”

Earlier, Mr McDonnell stressed that there would be "consequences" for MPs in his party who voted for Mr Johnson's plan but would not say whether that included having the whip removed.

The Shadow Chancellor said he would "have a chat" with Mr Campbell in a bid to persuade him to change his mind.

He was dismissive of suggestions a vote on a second referendum could take place on Saturday and said MPs should focus on defeating the deal.

This had been mooted as a possible amendment to the Government motion but some believe having an uncomplicated vote on the Johnson Plan would be the best way forward. The Liberal Democrats have already tabled a motion for a second referendum in next week’s vote on the Queen’s Speech.

Some amendments have already been tabled but it will be up to John Bercow, the Speaker, to decide on which ones or any he will call to be voted on.

The ones tabled so far include an amendment from Sir Oliver Letwin, one of the Tory rebels, who wants the Commons to withhold its approval of the deal until all the relevant legislation is passed.

SNP MP Angus MacNeil has tabled an amendment calling for the Government to revoke Article 50, while Ian Blackford, the Nationalist leader at Westminster, has tabled an amendment calling for an extension until at least January 31 to facilitate a general election.

The Commons debate will begin at 9.30am in the first Saturday sitting since the Falklands War in 1982. It is due to end mid-afternoon.