Boris Johnson is in a race against time to “ram through” his Brexit Bill at Westminster in just three days as Whitehall sources hinted he could pull it completely if MPs backed a second EU referendum or a customs union.

The Prime Minister is hoping to complete all the legislative stages in the House of Commons by Thursday evening. It looks set to involve MPs debating and voting through the night.

The plan is then to drive the bill through the House of Lords over the weekend, completing the process by early next week, so that the European Parliament can ratify the deal and ensure Britain leaves the EU, as planned, on October 31.

The parliamentary battle begins later today with the first key debate on the Withdrawal Bill, the Second Reading, when MPs will discuss and vote on the main principle of the legislation. Downing St believes the Government is on course for a narrow single-figure majority.

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The SNP was quick to table an amendment opposing giving the legislation a Second Reading, saying this was because “62 per cent of voters in Scotland supported remaining in the EU in the 2016 EU Referendum,” and adding the UK must not leave the Brussels bloc “before gaining the legislative consent of the Scottish Parliament”.

“Any attempt to bypass the devolved administrations would once again fly in the face of the rhetoric that we are in a so-called ‘partnership of equals,’” declared Ian Blackford, the party’s leader at Westminster.

As the Government published its 110-page bill Nicola Sturgeon complained of the lack of time for proper scrutiny. "Trying to ram through legislation of this complexity, significance and long-lasting consequences in just three days is an abomination of scrutiny and democracy," declared the First Minister.

However, if Mr Johnson and his colleagues do succeed in getting the bill a Second Reading, they must then try to win a crucial vote on the so-called “programme motion,” that sets out the Government’s time limits on various parts of the bill.

Jacob Rees-Mogg, the Commons Leader, made clear to MPs that losing this vote would mean completing the legislative process in the Commons and the Lords in time for Britain to leave the EU with a deal by the Hallowe’en deadline would be impossible.

"People who do not vote for the programme motion will not be voting for Brexit on October 31," he declared.

The warning appeared to be having an impact as last night some Labour MPs in Leave constituencies were said to be considering voting with the Government to ensure the legislative process can be completed to deliver Brexit by the October 31 deadline.

The next key part of the parliamentary process will be the committee stage, which will take up most of the time, when amendments are tabled and votes taken.

The two flashpoints are expected to be votes on Britain staying in the EU customs union and the holding of a second referendum on the Government’s deal versus Remain; what Whitehall insiders have dubbed “wrecking amendments”.

At the weekend, Dominic Raab, the Foreign Secretary, made clear the Government had “got the numbers” to get its deal through unamended. But others are not so sure.

In April when there were a series of indicative votes on various options, the one on a customs union was the closest to getting through; it was defeated by just three votes. The option of a confirmatory vote was defeated by 12 votes.

However, a lot of water has passed under the parliamentary bridge since then and some MPs who supported either option then have since changed their position.

One Tory backbencher said: “There’s a feeling we want to get this done.”

Plus, the prospect of the Government withdrawing the entire legislation is likely to concentrate the minds of several Conservatives who backed both options in the spring.

Downing St admitted: “If, essentially, the legislation in the House of Commons steps too far away from what was agreed in the Withdrawal Agreement and Political Declaration that does bring into question ratification.”

Meanwhile, Steve Baker, the European Research Group Chairman, urged his Conservative colleagues to reconsider backing the bill if it were "wrecked by opponents".

Some agonising was said to be going on in SNP ranks about the party’s approach to a second referendum; political opponents claim to have noticed a distinct cooling of late on the option by some Nationalist MPs.

But The Herald was told by senior SNP sources that the party, while it was likely not to vote for a customs union option because it would not deliver the full package - ie it lacked the single market aspect – it could vote for a second referendum, depending on the wording of the amendment. “It’s very possible we will back a second referendum,” said one SNP MP.

Ahead of the first Commons debate and vote later today, Mr Johnson said: “We have negotiated a new deal so that we can leave without disruption and provide a framework for a new relationship based on free trade and friendly co-operation. We are leaving the European Union but we will always be European.

“I hope Parliament today votes to take back control for itself and the British people and the country can start to focus on the cost of living, the NHS, and conserving our environment. The public doesn’t want any more delays, neither do other European leaders and neither do I.

“Let’s get Brexit done on October 31 and move on,” he added.

Earlier, as many had expected, John Bercow rejected a bid by ministers for a fresh “meaningful vote” on the Brexit deal struck last week with Brussels.

The Speaker ruled the special Commons sitting on Saturday had voted to delay approval until the implementing legislation had been passed and that any further vote would be "repetitive and disorderly" under House rules.

The decision by Mr Bercow, which he described as “perfectly reasonable,” did not go down well with Government ministers or Conservative Brexiteers. One, Sir Bernard Jenkin, complained how it was “becoming remarkable how often you please one lot and not the other lot".

In other developments:

*Ms Sturgeon in a letter to Donald Tusk, the European Council President, jointly penned with Mark Drakeford, her Welsh counterpart, appealed for Brussels to delay Brexit to allow time for another referendum to take place;

*Michael Gove, the Cabinet Office Minister, said a no-deal Brexit would result in direct rule being reintroduced in Northern Ireland as he said a new Government Brexit information campaign on no-deal planning would "reflect new urgency" in advertisements appearing from today;

*Chancellor Sajid Javid confirmed there were no economic impact assessments of the UK-EU Brexit deal, pointing out how ending the Brexit divisions "could not be measured solely through spreadsheets and impact assessments" and

*the European Parliament indicated it would not ratify the Brexit deal this week but would wait for Westminster to pass the legislation with a view to a possible extraordinary session next week.