LONDON, United Kingdom (AFP) — Britain’s government cannot submit its Brexit deal for another vote in parliament if it is “the same” or “substantially the same” as the one already rejected by MPs, the House of Commons speaker said on Monday.

“What the government cannot legitimately do is to re-submit to the House the same proposition or substantially the same proposition,” John Bercow told parliament.

The move presents a problem for Prime Minister Theresa May, who was expected to bring her unpopular deal back for a third time ahead of Thursday’s EU summit, where she is to ask for a delay to the March 29 Brexit date.

It is the first time in more than 100 years that the speaker has invoked the rule, meaning the government must find a way of substantially altering the bill before it can be voted on.

“This convention is very strong and of long standing, dating back to 2 April, 1604,” said Bercow, who added it was last used to stop a repeat vote in 1912.

The speaker, who has been accused of bias during the Brexit process, said he allowed the deal to return to parliament last week, despite its defeat in January, as it “contained a number of legal changes” from the original.

“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,” said the speaker.

May’s Downing Street office said it had no comment on Bercow’s announcement because “we weren’t given any advance warning.”

The convention applies to motions submitted in the same parliamentary session. Brexit-supporting MP Jacob Rees-Mogg asked about the possibility of a parliamentary prorogue.

That would require May to ask Queen Elizabeth II to break up parliament early, before reconvening in a new session.