Nowadays, most of the 781 members of the upper chamber, known as peers, are political appointees, rewarded for a career in politics or public service with a lifetime place in the Lords that brings status and influence — as well as a desk and parking space in central London, and a daily attendance allowance of 300 pounds, about $400.

Over the years the numbers grew (before falling back a little recently), as prime ministers secured seats for allies who have mostly done low-key work, revising legislation or sitting on expert committees.

But with Britain still bitterly divided over the merits of Brexit, the House of Lords has focused attention on political weak spots in Brexit plans, particularly over moves to abandon a customs union with the European Union, an issue so contentious that it has divided and paralyzed the cabinet.

Lords have proposed numerous amendments to the Brexit bill. But experts believe that, if the Commons overturns those amendments — as the government hopes it will — the Lords will in any case back down, as custom dictates, even before reaching the legal limits of its delaying power.

“I don’t think they are doing anything constitutionally dubious,” said Prof. Meg Russell, director of the Constitution Unit at University College London. “There is nothing that the Lords has done that is a direct challenge to the principle of Brexit.”

“All the House of Lords ever — or nearly ever — does is essentially to throw the question back to the House of Commons and say: ‘Are you sure about this?’ ” she added.

The 2016 referendum approved withdrawal in principle but left the details to the government and Parliament, and the amendments proposed by the House of Lords have stayed within its powers by focusing on these, Professor Russell said.