The Prime Minister will speak to European Commission president Jean-Claude Juncker this morning to reassure him she still intends to trigger Article 50, the formal process for cutting ties with Brussels, by the end of March.

Meanwhile, acting UKIP leader Nigel Farage has added his voice to calls for Mrs May to call an emergency General Election following the legal setback.

He said she should go to the country even though such a move would be strongly opposed by Conservative and Labour MPs alike.

Writing in the Daily Telegraph, Mr Farage said: "It is unlikely that Labour MPs, led by a man they don't believe in, would be keen on facing the likely electoral oblivion that would follow.

"It also seems likely that some on the Prime Minister's own backbenches, who are seeking to overrule the will of the British people, would be unlikely to support an early General Election either."

He went on: "The establishment, in denial after the referendum result, still just don't get it.

"The British people are not simply going to let this incredible establishment arrogance lie. I suspect even more radical political change is on the way."

The Government immediately announced it will appeal against the High Court ruling and take the matter to the Supreme Court in early December.A Downing Street spokeswoman said Mrs May was keen to tell other European leaders she would stick to the declared timetable for withdrawal.

She added: "She will set out what the process is, which is that we are appealing and that we are carrying ahead and sticking to the timetable we have set out. 

"This judgement is not going to derail that."

But Tory peer Baroness Wheatcroft told the BBC the March deadline for triggering Article 50 now looked like "an impossible target".

In a sign the Government could face difficulties getting legislation through the Lords, she said: "I think that it's only right to delay triggering Article 50 until we have a clearer idea of what it actually entails and I think there will be others in the Lords who feel the same way."Gina Miller, the businesswoman who brought the case with hairdresser Deir Dos Santos, welcomed the outcome, saying it would "bring sobriety" to Brexit proceedings.

Opposition leaders Nicola Sturgeon, Tim Farron and Jeremy Corbyn, who have been calling for Mrs May to lay out her Brexit strategy more clearly, also welcomed the ruling.

Labour leader Mr Corbyn said: "(It) underlines the need for the Government to bring its negotiating terms to Parliament without delay.

"Labour respects the decision of the British people to leave the European Union. But there must be transparency and accountability to Parliament on the terms of Brexit."

Ms Sturgeon said Scotland could join the Brexit court battle to make sure Mrs May does not win back the power to trigger Article 50.