Prime Minister Theresa May's government has suffered its 15th defeat on legislation that will end Britain's membership of the European Union when parliament's upper chamber voted in favour of adding environmental safeguards to the bill.
May has to get the bill approved by both chambers of parliament well in advance of Britain's exit on March 29 in 2019, but the House of Lords, the unelected upper house, has demanded major changes that will force a showdown over coming weeks.
The Lords voted 294 to 244 in favour of a change to the bill which would force the government to maintain the EU's environmental principles.
The government argues that Brexit will allow Britain to improve environmental protections through separate legislation.
May's Conservative government has already suffered high-profile defeats on core Brexit issues such as whether Britain should leave the EU's single market and customs union.
Martin Callanan, a junior Brexit minister who is a member of the House of Lords, said the government had listened to constructive suggestions to improve the legislation which is now due to return to the lower house.
"However, during the bill's journey through the House of Lords some changes have been made that conflict with its purpose or are designed to frustrate the entire exit process and so we are considering the implications of those decisions," he said.
While the more powerful House of Commons can overturn the changes, they may embolden rebels in May's own party who favour a softer EU exit.
Ministers have accused the House of Lords, where the Conservatives do not have a majority, of making unnecessary changes and have indicated they will fight some of them back in the Commons.
That process, known as 'ping pong', is not yet scheduled, but will be a key test of May's ability to govern effectively and to deliver on her Brexit plans with just a slim working majority in the Commons, where she relies on the support of a small Northern Irish party.
