Britain's unelected House of Lords inflicted a major defeat Monday for Prime Minister Theresa May with a landslide vote in a debate on the government's Brexit bill.

The vote came after the lords debated an amendment over the wording of what would be a meaningful vote on a final Brexit deal with the European Union (EU).

May and her senior Brexit ministers were eager to avoid words in the Brexit bill that would tie their hands in their negotiations with Brussels.

The House of Lords voted by 354 votes to 235 in favor of an amendment that had been worded by pro-EU Conservative MP, the former Attorney General Dominic Grieve.

It will mean a new cliffhanger when MPs vote on Wednesday to decide how much of a say the British Parliament should have on a Brexit deal.

Media reports in London said more than 22 Conservative peers voted against May's government in what will be seen as a massive blow for May and her government.

Grieve said later the government went back on an agreement reached with May last week when he supported the government in the House of Commons during the debate on the Brexit withdrawal bill. He said the compromise reached was changed and made it unacceptable.

Lord Hailsham, who led the anti-government rebellion in the Lords said he was asking members to make a decision to enable a House of Commons to vote on what Grieve believed had been agreed with the government last week. He described Brexit as a national calamity for Britain.

Hailsham said the government's offer not only failed to deliver a promised meaningful vote but was worse.