British Prime Minister, Theresa May defeated again over Brexit-trigger law

British Prime Minister, Theresa May. Photo / AP

British Prime Minister Theresa May suffered a fresh blow to her plan to trigger the start of Brexit talks after the upper-house demanded the power to reject the final deal she reaches with the European Union.

The House of Lords in London voted by 366 to 268 today in favour of changing May's draft Brexit law to give Parliament the ability to send her back to the negotiating table if legislators decide the terms of the UK's exit agreement aren't good enough.

Rebels in May's Conservative Party joined forces with Labour and Liberal Democrat members of the upper chamber to defeat the Government. It is the second time they've defied the PM on amendments to the bill giving her power to formally notify the EU that Britain is leaving the bloc. Another change to the wording, passed last week, aims to protect the rights of EU citizens living in Britain.

Last year's referendum vote to leave the EU was an instruction to government ministers to negotiate exit on "the best terms" they can get, not to leave "whatever the cost," former Tory cabinet minister Douglas Hogg told members during the debate.

Dianne Hayter, speaking for the opposition Labour Party, said that without the amendment, it would be mandatory for the EU Parliament to give its consent to the eventual Brexit deal, but not for the British legislature."This country's future should rest with Parliament and not with ministers."

"It's about authorising Parliament," she said. "It's to put wheels on the outcome of the referendum."

May's defeat means that she'll either have to accept the changes - which she's argued would weaken her in negotiations - or try to delete the amendments. Her team has signalled she's likely to try to overturn the Lords defeats when the bill returns to the House of Commons for further consideration, probably next week.

The PM wants to open Brexit talks by the end of this month by formally invoking Article 50 of the bloc's Lisbon Treaty. May hasn't named a date for making the move and there are conflicting reports on when she might pull the trigger, ranging from March 15 to March 31.

Today's amendment guarantees Parliament a veto over the final Brexit agreement, as well as the power to stop May walking away from talks without a deal, something she's threatened to do if she doesn't like the EU's offer.

May wants the 137-word draft to go through Parliament without changes to give her the maximum freedom to negotiate with EU leaders. Her spokesman, James Slack, told reporters that the PM is "absolutely" on course to trigger Brexit negotiations by the end of March.

Brexit Minister George Bridges told the Lords that while the Government has already promised them a vote on the eventual Brexit deal, enshrining that promise in the Brexit bill "could create a lucrative field day for lawyers" and hamper the UK.

"This amendment simply makes the negotiations much harder from day one for the prime minister," Bridges said. "It increases the incentive for the European Union to offer nothing but a bad deal."

May's office confirmed that the vote she's planning to give Parliament without the amendment would simply be one on whether to accept the deal or to leave without a deal.