The unelected upper chamber of the UK parliament voted 366 to 268 on Tuesday to require approval from parliament, not just the government, on the final deal for Britain to leave the European Union and future trade ties.

It's an amendment to the bill which was drafted to empower Prime Minister Theresa May to trigger Article 50 of the EU's Lisbon Treaty. Triggering Article 50 starts the clock ticking on the two-year process for the UK and the bloc to disentangle themselves from one another. May had promised to begin the withdrawal process by the end of March.

The timeline for her to do so is already about a week behind schedule as the House of Lords also voted on March 1 for another amendment which would guarantee the rights of the estimated 3 million EU citizens living in the UK, but May remained confident she would still meet her goal. Her government said the amendments would damage their standing in Brexit negotiations.

"It is clear that some in the Lords would seek to frustrate that process, and it is the Government's intention to ensure that does not happen. We will now aim to overturn these amendments in the House of Commons," Brexit Minister David Davis said in response to the vote. 

Parliamentary ping pong

The vote was seen as a blow to the Conservative government as it signals a possible stand-off between the unelected House of Lords and the elected lower chamber, the House of Commons.

In many cases amendments to bills made by one house need to go back to the other for approval in a potentially time-consuming process known as "parliamentary ping pong." In this case, the Commons is likely to overturn the amendments made by the Lords.

May had told the Lords repeatedly not to amend the short bill which she says was designed only to implement the referendum result.

"She's been clear that she wants an unamended bill to pass. This is a simple bill with one purpose - to give the government the power to trigger Article 50," her spokesman told reporters on Tuesday.

Additional amendments are also viewed as a further sign of the domestic opposition Theresa May could face as she negotiates Brexit, which was voted for by 51.9 percent of voters in a June 2016 referendum.

Tough negotiations expected

EU leaders have indicated that the UK will not be given an easy ride during the negotiations process, partly due to fears that Brexit could set a precedent for the bloc's 27 other member states.

"Now it's important that we in the EU-27 stick together," German Finance Minister Wolfgang Schäuble told reporters in Berlin, adding that at the conclusion of the negotiations it should be clear to everyone that remaining in the EU had more advantages than leaving it.

May has expressed optimism about the deal the UK will be able to reach with the EU. However, at least in theory, if a deal is not reached by the time the two-year deadline laid out in Article 50 runs out, the trade deals and contracts between Britain and the bloc would be void overnight. In practice, however, London and Brussels alike are stepping into uncharted waters.

se/msh (AFP, AP, Reuters)