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Speaker John Bercow has dramatically BLOCKED Theresa May from bringing back her Brexit deal for a third meaningful vote in the House of Commons, unless the Government makes "substantial" changes.

In a bombshell for the Prime Minister, he said Commons rules say the same motion "in substance...may not be brought forward again during that same session."

In an unexpected statement in the House of Commons, Bercow cited regulations dating back to 1604.

And Number 10 said they were not given advance warning of the Speaker's statement.

It's led to fears the Government could try to force another vote through by ending the current Parliamentary session and starting a new one.

That could result in dozens of laws - many of which would be required for Brexit to happen - being scrapped.

Mrs May's spokesman told reporters he was not yet in a position to comment on the statement, when John Bercow said the government would have to bring a different proposition to parliament if it wanted to stage another Brexit vote.

He said: "Decisions of the House matter. They have weight. In many cases they have direct effect not only here but on the lives of our constituents.”

He said bringing the same deal, or substantially the same deal, back to the Commons would not be "proper".

(Image: AFP/Getty Images) (Image: REUTERS)

He said: "If the government wishes to bring forward a new proposition that is neither the same nor substantially the same as that disposed of by the House on March 12, this would be entirely in order.

“What the government cannot legitimately do is to re-submit to the House the same proposition or substantially the same proposition as that of last week that was rejected by 149 votes.”

Labour's Hilary Benn asked the speaker to set out how much the motion would have to be changed in order to count as a new motion.

Bercow said: "I do think a demonstrable change to the proposition would be required, for example, simply a change in an opinion on something wouldn't itself constitute a change in the offer.

"And so I would have to look at the particulars, I would have to make an honest assessment of the circumstances and perhaps of the competing claims made as to the veracity of one proposition or argument or another.

"But fundamentally for something to be different it has to be fundamentally different."

Downing Street has been scrambling for support for the deal - particularly from the Democratic Unionist Party (DUP) - in the hope of bringing it back before Parliament ahead of an EU summit on Thursday.

But it is thought the Prime Minister planned to delay the crucial vote for another week unless she was confident of avoiding a third humiliating defeat on the package, which MPs rejected by 230 votes in January and 149 last week.

She has warned that if her deal is not approved, the UK will have to seek a lengthy extension to negotiations, potentially losing Brexit altogether.

The Prime Minister's official spokesman confirmed ministers would want to be confident they had a "realistic prospect" of success before deciding to call a third vote.