UKIP Leader Paul Nuttall has warned that Britain will be dragged into its most serious constitutional crisis for over a century if the House of Lords tinkers with Brexit.
He said: "What we have here is the least democratic forum in British politics potentially interfering with the most democratic one - the result of a referendum.
"The British people have made their choice. For the Lords - which is stuffed with pro-EU timeservers and from which UKIP has been systematically excluded - to mess around with it would be both outrageous and dangerous.
"We would in my view be in a situation akin to that before the First World War when the Parliament Act was passed to bring the Upper House to heel after the Lords interfered with a finance Bill.
"The arrogance of Labour peers such as Lord Mandelson and Lord Hain is breath-taking. Unelected figures who set themselves against the will of the people are being grossly irresponsible.
"I cannot predict the ferocity of the response we might see if our democracy is subverted in this way. I urge pro-EU peers not to go down this perilous path.
"In the last great constitutional crisis David Lloyd-George called the Lords 'Mr Balfour's Poodle' and referred to peers as 500 men drawn from the ranks of the unemployed.
"I note the relevant figure is now 900 men and women, which does not look like progress to me. The Lords must not now become Peter Mandelson's Poodle."