Parliament will amend and delay Brexit unless an acceptable withdrawal plan is put forward, Theresa May has been warned.

Both Nick Clegg and a Conservative peer seized on yesterday’s High Court ruling to tell the Prime Minister she no longer had a free hand in the exit talks.

The former Liberal Democrat leader said the party’s MPs and peers would demand a soft Brexit and – crucially – a second referendum on the deal that emerges.

Meanwhile, Tory peer Baroness Wheatcroft said the House of Lords was ready to stall the legislation now likely to be needed to trigger the Article 50 notice period, if necessary.

The comments will infuriate hardline Brexit supporters, who have already accused the High Court judges of trying to block the referendum result.

Theresa Villiers, the former Conservative cabinet minister, said it would be a “constitutional outrage” if unelected Lib Dem peers stood in the way of Brexit.

And, last night, Local Government Secretary Sajid Javid said there was a “moral issue” at stake in the battle over EU withdrawal.

He turned on the people who brought the case that won in the High Court, saying: “This is an attempt to frustrate the will of the British people and it is unacceptable.”

Mr Clegg and Baroness Wheatcroft spoke out after the Prime Minister accepted that an act of Parliament will be necessary to win the right to trigger Article 50 – unless the Supreme Court overturns the judgment.

That would open the door for Parliament to amend – and even delay – the Government’s strategy for withdrawal, perhaps in lengthy Parliamentary battles.

Landmark Brexit Ruling: What happened and what happens next?

It throws into doubt Ms May’s timetable for invoking Article 50 by the end of March – opening up speculation about a snap election to break through the impasse.

Mr Clegg told Radio 4’s Today programme: “We will seek, with other parties in both the House of Commons and the House of Lords, to amend the legislation such that Parliament would say to the Government that it should pursue a soft Brexit and not a hard Brexit – and that there should be some means by which the British people can have a say on the final deal when the negotiations with the European Union are finally completed in the years ahead.”

Mr Clegg said he believed there was a “body of opinion” across the various parties in Parliament to try to rein in Ms May in this way.

He added: “If we were to be able to marshall opinion behind that approach then people will vote in favour of Article 50 triggered on that basis.”

He warned the Government not to “dig its heels in” – by pursuing a hard Brexit and refusing a second referendum at the end of the negotiations.

“Then of course, I think people will say, well hang on a minute, we are not sure if we are going to give you the consent to proceed on that basis,” Mr Clegg said.

A soft Brexit is normally taken to mean continued membership of the EU’s single market trading arrangements – rather than making curbs on immigration a “red line”.

Baroness Wheatcroft told the same programme that still trying to invoke Article 50 by the end of March was now an “impossible target”.

She said: “I think it’s only right to delay triggering Article 50 until we have a clearer idea of what it actually entails

“I think there will be others in the Lords who feel the same way.