DOWNING St has admitted that any significant change to the UK Government’s Brexit Bill would “bring into question” the ratification of the departure deal agreed with the EU.

In such circumstances, say, if MPs successfully amended the Withdrawal Agreement Bill to include a second referendum on the deal or joining a customs union with the EU27, then it is possible, if not probable, Boris Johnson would pull the bill.

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

No 10 also made clear that this afternoon if John Bercow, the Commons Speaker, grants another so-called “meaningful vote” on the principle of the UK-EU deal and opposition parties put down amendments to it, the Government would withdraw the motion, which it believes would then have become “meaningless” as it wants a straight yes-no verdict on the agreement with Brussels.

As the Prime Minister and his ministers prepare for yet another crucial week at Westminster, they face a race against time to get the Brexit bill through both Houses of Parliament by the middle of next week.

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The Withdrawal Bill is set to be published this evening so MPs can have a first debate and vote on it, at Second Reading, tomorrow. There will also be a vote on a programme motion, whereby ministers will set out how it intends to use parliamentary time to push through its legislation. A defeat here would make the Government’s plan to get the bill through both Houses by next week virtually impossible.

Government whips intend to make MPs sit through the night to get all the legislative stages through the Commons by the end of the week and then will make the Lords sit over the weekend to enable the Brexit deal to become law by the beginning of next week when the European Parliament will hope to ratify it.

However, with opposition MPs lining up to table a raft of amendments the Government believes there will be concerted attempt to sabotage the bill.

Asked, if MPs successfully changed it to introduce a second referendum or a customs union, would the Government withdraw it, Mr Johnson’s spokesman said: “The Government is opposed to both the customs union and a second referendum…Both of those have been voted on on a number of occasions in the Commons and neither have been successful.”

Asked the question again, he pointed out no amendments had yet been tabled, so the conversation could be picked up if and when that happened. The spokesman repeated the point that MPs had explored both issues before and neither had been voted for.

When it was suggested any significant change to the bill would mean the agreed deal with the EU would not have been ratified, the spokesman replied: “You are correct in saying that 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.

“The point of the Withdrawal Bill is to put into domestic law the international agreement we have reached with the EU,” he added.

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Concern has been raised in Tory circles that Northern Ireland’s Democratic Unionists, while opposed to a second referendum, might be prepared to back a proposal for a customs union as this would remove any border down the Irish Sea, which is the effect of the Johnson Plan and which they are bitterly opposed to. One party source admitted the customs union option, previously opposed by the DUP, might now be the only way to ensure the whole UK leaves the EU under the same arrangements.

The UK Government is fervently opposed to a customs union with the EU as it would prevent the UK Government, post-Brexit, from signing its own international trade deals.

At the weekend, Labour’s Keir Starmer invited the DUP to meet to discuss amendments to the legislation, stressing his party’s “door is open”.

As Westminster wades through the gruelling parliamentary process, Brussels will watch to see if it will need to step in to offer an extension. Diplomatic sources have suggested the EU would be prepared to delay Brexit until February should Mr Johnson find it impossible to get the deal past MPs this week.