John Bercow has ruled out another vote on the government’s previously rejected Brexit agreement if the motion remains “substantially the same”.
He told the Commons that parliamentary conventions dating back to 1604 meant MPs could not be asked to vote on precisely the same subject twice.
MPs rejected Theresa May’s Brexit deal with the EU by 149 votes last week.
Mr Bercow’s ruling came as the government considers a third attempt to get the deal through Parliament.
He said the second vote on the prime minister’s deal last week was “in order” but any further votes must pass this “test” in order to be allowed.
The BBC’s Laura Kuenssberg said the Speaker’s intervention could have a “massive” impact on the Brexit process – with 11 days to go before the UK is scheduled to leave the EU on 29 March.
Mrs May had previously committed to holding a new vote on her withdrawal agreement in the coming days, but ministers have said this will not happen unless they are “confident” of victory.
The prime minister’s official spokesman said Mr Bercow “did not warn us of the contents of the statement or indeed the fact that he was making one”.
Mr Bercow made his ruling in response to what he said were MPs’ concerns that the government would seek “bring the same deal back to the House ad infinitum” and ask them to “repeatedly pronounce on fundamentally the same proposition”.
He said last week’s second vote on the deal had not fallen foul of parliamentary conventions.
The government, he said, had secured “legal changes” to its agreement with the EU and what MPs had been asked to consider was different from the agreement rejected by 230 votes in January.
But quoting Erskine May, the parliamentary rulebook, he said if the government attempted to bring back “the same or substantially the same motion” in the coming days then he would intervene to stop it.
The convention, he added, was “necessary to ensure the sensible use of the House’s time and the proper respect for the decisions it takes”.
Brexiteers welcome ruling
Opponents of the PM’s Brexit deal welcomed the Speaker’s ruling.
Conservative former cabinet minister Owen Paterson said it was a “game-changer” and would “concentrate minds” ahead of a EU summit on Thursday – at which EU leaders will consider to grant the UK an extension to the Brexit process.
He said the prime minister should make it very clear to her European counterparts that the “law of the land” stated the UK would leave, with or without a deal, on 29 March.
Sir Bill Cash, chairman of the European Scrutiny Committee, said it seemed to make an “enormous amount of sense” given that the Brexit deal has been defeated twice and there would need to be a “substantial difference” to allow a third vote.
But the SNP’s Westminster leader Ian Blackford suggested there was now a “constitutional crisis” and he suggested the prime minister should “immediately” call a meeting of opposition leaders.
Third Brexit vote must be different – Speaker}