THERESA May will face a parliamentary battle to get her Brexit Bill through the House of Lords as Labour launched a renewed effort to rewrite the legislation.

This week the Commons overwhelmingly backed legislation allowing the Prime Minister to begin Brexit without altering the bill but the Government will face a stiffer test in the Lords where the Conservatives do not have a majority.

If peers attempt to hold up the legislation or seek to dramatically alter it, a full-blown constitutional crisis could develop, with one Government source warning that the public outcry could lead to the abolition of the unelected chamber.

However, Downing Street sought to play down such talk and stressed how the Government regarded the second chamber as having an “important constitutional role” to scrutinise legislation.

Nonetheless, Brexit Secretary David Davis has warned peers to do their "patriotic duty" and back the bill.

Labour's frontbenchers responded by insisting they would not be "cowed by threats" and tabled a series of eight amendments, including a demand for greater certainty over the promised vote on the Brexit deal and a fresh call for the rights of EU citizens in the UK to be protected.

Mrs May wants to trigger Article 50 by the end of March. It is thought that given the Lords will have its final vote on the legislation on March 7, barring any Government defeats, then she could formally begin the process at the end of that week after the Budget.

Leaving it to the end of the month would be regarded as difficult, given the 25th marks the 60th anniversary of the Treaty of Rome, which established the European Economic Community, the forerunner of the EU.

During a press conference after a bilateral with Paolo Gentiloni, the Italian Prime Minister, he warned Mrs May that the Brussels negotiations would “not be easy" but insisted the process must not become "destructive".

“Obviously, we will do this in the hope of fostering the unity of the 27 countries because without the unity of the 27 countries, it will be difficult to come to some agreement. We must ensure this unity will result in the best possible agreement with the UK," said Mr Gentiloni.

Mrs May underlined plans to make guaranteeing the status of EU nationals living in the UK and those of Britons living in the rest of the bloc a top priority once talks finally begin.

She said: "There is goodwill on all sides in relation to this matter. We recognise people want reassurance for their future."

Among those peers already listed as due to speak in the first Lords debate, the Second Reading, on Monday February 20 are former Labour Chancellor Lord Darling of Roulanish, former Conservative Chancellors Lords Lamont and Blaby and former Liberal Democrat leader Lord Campbell of Pittenweem.