The businesswoman behind the Article 50 court case has threatened another legal challenge unless Theresa May guarantees a 'meaningful vote' on any new deal with the EU.

Gina Miller said she could not think of 'anything better to do' with her money than ensuring the PM brings the package back before the Commons.

The comments came after campaigners obtained legal advice that Britain's 'actual withdrawal' will have to be authorised by a fresh act of Parliament.

Ms Miller, who infuriated Brexit supporters by challenging Mrs May in court, said the Government would have to be 'incredibly careless' to ignore the law a second time.

Gina Miller said she could not think of 'anything better to do' with her money than mounting another legal challenge to the government

The House of Lords could inflict a second defeat on Mrs May over the Brexit Bill on Monday by re-writing the draft law to demand a meaningful vote on the final deal.

If the amendment either fails or is defeated by MPs, Mrs Miller warned it could prompt a new legal challenge.

She told the Independent: 'Would it not be easier for the Government to accept the amendment for a vote at the end of the negotiations in 18 months' time now, rather than facing legal challenges in 18 months' time?

'The Government tried to ignore the law once before, doing it twice would be incredibly careless.'

In a separate interview with BBC Radio 5 Live Daily she said: 'It is my money and my decision and my actions, and I can't think of anything better to do with the success I have.'

The government suffered its first defeat over the Brexit Bill last night when the House of Lords backed an amendment ordering Mrs May to guarantee the future status of EU nationals already in the UK.

WHAT HAPPENS NEXT FOR THE BREXIT BILL? The Lords finished the 'committee stage' of the crucial legislation last night. After another detailed debate, peers will give the Bill its 'third reading' next Tuesday and send it back to the Commons. At that point the government will have to consider whether to accept amendments that have been passed - or get MPs to overturn them. That could trigger a bout of 'ping pong' - where disputed legislation is passed back and forth between the Houses. Advertisement

Downing Street immediately made clear that the PM would seek to overturn the vote when the legislation goes back to the House of Commons later this month.

However, they are facing the prospect of a second setback in the Lords next week on an amendment that would require a 'meaningful' vote on Britain's final withdrawal.

While Mrs May has promised a vote on any agreement with the EU, Remainers fear it will only be held when it is too late to return to the table and renegotiate.

The Open Britain campaign group has sought to stiffen opposition in the Lords by circulating details of legal advice it has obtained from three leading QCs - Sir David Edward, Sir Francis Jacobs and Sir Jeremy Lever.

According to the group, the advice states 'actual withdrawal' will require a 'full act of Parliament' once the terms of any deal are known and that legislation would also be required if the Brexit negotiations ended without an agreement.

Theresa May has pledged to trigger Article 50 by the end of this month despite the stinging defeat in the House of Lords

The House of Lords tonight overwhelmingly passed an amendment to the crucial legislation that would force the PM to make a commitment before a deal is struck with Brussels

Labour MP Chuka Umunna warned ministers could face another court battle unless they were prepared to amend the current legislation.

HOW GINA MILLER ENTERED THE BREXIT BATTLE June 2016: Brexit vote secured at the EU referendum June 2016: Gina Miller lodges legal action questioning whether Theresa May can invoke Article 50 without Parliament passing a new law. November 2016: The High Court defies expectations and backs Miller in an embarrassing defeat for the government. May vows to appeal. November 2016: In the aftermath of the result, Miller is subject to a torrent of abuse, prompting arrests to be made. December 2016: All 11 Supreme Court Justices consider the issue in an historic constitutional law case that lasts four days. During the hearing, a symbolic Commons vote is held endorsing the triggering of Article 50. January 2016: Supreme Court judges uphold the High Court ruling. May vows to press ahead with her plans to invoke Article 50 by the end of March. February 2016: Legislation for Article 50 is rushed through the Commons un-amended in a landmark series of votes. Advertisement

'It is completely unacceptable for the Government to only offer Parliament a 'bad deal or no deal' ultimatum at the end of the negotiations,' he said.

'The Government should accept the amendments that give Parliament a meaningful vote and a real choice at the end of this process.

'That's the democratic and patriotic thing to do. We don't want any more time, effort or taxpayers' money spent on court cases.'

The threat of further legal action will cause fury among Brexiteers about attempts to derail the process of leaving the EU.

Brexit Secretary David Davis has insisted the government is 'on track' to invoke Article 50 by the end of this month.

'The most important thing in this is that we will conclude in time to present the Article 50 letter and will do so in terms that are in the interests of both ourselves and the European Union,' he told journalists during a visit to Copenhagen today.

In other developments today, the head of the CBI warned leaving the European Union without a deal on future trade arrangements would be 'not only wrong but irresponsible'.

Paul Drechsler, the CBI president, says that if the UK is left to fall back on World Trade Organisation (WTO) rules after Brexit it will open up a 'Pandora's Box' of unforeseen consequences for the economy.

Mrs Miller spearheaded the legal challenge that forced the government to pass legislation before the PM triggers the Article 50 process

In a keynote speech in the City of London, Mr Drechsler will warn that British firms exporting to the EU would face tariffs of 90% on their products without an agreement as well as a raft of new regulatory hurdles to negotiate.

While some businesses were already preparing for such a 'worst case scenario', others were unable to do so because the costs were too high.

With Theresa May on course to invoke Article 50 marking the start of the formal process of withdrawal from the EU by the end of the month, Mr Drechsler will highlight the deep unease among the business community over what happens next.

'Right now, it feels like we're just reaching the top of the Article 50 rollercoaster.

'Any minute now, maybe next week, maybe the week after that we'll suddenly drop into the twists and turns of negotiations,' he will say according to advance extracts of his speech .

Ms Miller's legal challenge infuriated supporters of Brexit and she has clashed publicly with Nigel Farage, pictured

Businesses on both sides of the Channel, he will say, are concerned at the implications for future trade if the Government is unable to reach a deal with the remaining 27 member states.

'We should be under no illusions about what this would really mean. A 'no deal' scenario would open a Pandora's Box of economic consequences,' he will say.

'Here in the UK and across the continent firms are worried about this 'worst-case scenario'. Some are getting ready for it to reduce economic damage.

'Some won't prepare because they're hoping for a deal. But in reality many firms can't prepare because the cost of change is simply too high to even consider it.

'The Prime Minister is confident that a deal can be achieved - and we agree. But to those whose first and only choice is for Britain to walk away without a deal, I say you're not only wrong but irresponsible.'