Speaker John Bercow has rejected the prospect of Theresa May bringing her deal back to Parliament for a third time, unless the deal is substantially changed.
This intervention substantially increases the prospect of a General Election after it had been rumoured that the government could try and push the bad deal through before 29th March.
In the surprise ruling, he said he would not allow a third "meaningful vote" in the coming days on "substantially the same" motion as MPs rejected last week.
With only 11 days to go before the UK is due to leave the EU, ministers have warned of a looming "constitutional crisis".
Prime minister Theresa May has negotiated the withdrawal deal with the EU but it must also be agreed by MPs, who have already voted against it twice.
Mr Bercow cited a convention dating back to 1604 that a defeated motion could not be brought back in the same form during the course of a parliamentary session.
He said the second vote on the prime minister's deal last week was "in order" as it was substantially different to the first, but any further votes must pass the "test" he set out to be allowed.