LONDON (AFP) - Legislation empowering Prime Minister Theresa May to start Brexit negotiations moves to Britain's House of Lords on Thursday (Feb 9) - with ministers urging peers to do their "patriotic duty" and back it.

MPs in parliament's lower chamber, the House of Commons, overwhelmingly backed the bill late on Wednesday to allow May to trigger Article 50 of the EU's Lisbon Treaty, which she has promised to do by the end of March.

Downing Street played down a warning by an unnamed government source that the unelected second chamber could face abolition if it stands in the way of Britain's exit from the European Union.

But Brexit minister David Davis told Sky News: "I expect it to do its job and to do its patriotic duty and actually give us the right to go on and negotiate that new relationship (with the EU)."

The source had earlier told journalists that the Lords, which many believe has long been ripe for reform, "will face an overwhelming public call to be abolished if they now try and frustrate this bill".

May's Conservative party does not have a majority in the House of Lords, raising the possibility that peers may seek to block Brexit.

However, the opposition Labour leader in the chamber, Baroness Angela Smith, said in October that she would not be obstructive.

"We will scrutinise; we will examine; we will not block," she said.

Britain's upper house, with 805 members, is the second-largest parliamentary chamber in the world, beaten only by China's National People's Congress.

The European Union (Notification of Withdrawal) Bill passed the Commons by 494 votes to 122 on Wednesday and after parliament takes a break next week, will be debated on the Lords starting on February 20.

Ministers hope it will be approved on March 7.