Robin Tilbrook, pictured, has launched a High Court battle claiming that the UK has already left the EU

The leader of the English Democrats has launched a High Court battle claiming that the UK has already left the EU.

Robin Tilbrook argues that Britain left the union on March 29 as originally planned, saying the PM's extension was not legally valid.

The legal bid will be seen as a Brexiteer equivalent to Gina Miller's battle with the Government over triggering Article 50 in 2016.

Speaking to MailOnline last night Mr Tilbrook said the argument that Britain had already left was 'very strong'.

In the judicial review application, the English Democrats argue that Mrs May did not have the power to extend the exit date past March 29.

'Her purported request for an extension of the date of departure and the Government's purported agreement to such an extension is and was unlawful and is and was null and void,' the legal argument reads.

Mr Tillbrook hopes the case will be brought before a court in the coming weeks.

The case comes after a former appeal court judge suggested the way the extension was handled could have been unlawful, and a group of Tory MPs wrote to the Prime Minister asking to see the legal advice she received.

The English Democrats argue in their legal application that Theresa May (pictured last night) did not have the power to extend the exit date past March 29

Parliament's website asserts that the Commons and Lords agreed a legally valid extension to the original date.

Mr Tilbrook said: 'What we're hoping to achieve with it is a declaration that we're already out. If you've read it, you can see that the argument's very strong.

'We've done it all, we started a month or so ago. We've served all the paperwork in advance and we've issued it today.

'I fully expect it to be heard fairly soon. I've asked the High Court for it to be heard on an expedited basis. I think the argument's strong that we're out.

'The Government's proceeding on the basis that we're still in. We're saying that's not the case. If there's a declaration from the High Court that we're out, the Government's got a new reality to deal with.'

Mr Tillbrook says that, if a judge finds in his favour, he will save taxpayers Britain's £39billion divorce bill and end the Brexit impasse.

The government has yet to state its defence in the case, but wrote to Mr Tilbrook last month claiming he had failed to identify a decision capable of judicial review.

A spokesman for the Department for Exiting the European Union said today: 'As a matter of international law, the date of the UK's exit from the EU was changed following the decision of the EU Council held on 22 March and the exchange of letters between the UK and EU on 25 March. We reflected this change in domestic law when we made the relevant statutory instrument on Thursday 28 March.'

The legal bid will be seen as a Brexiteer equivalent to Gina Miller's battle with the Government over triggering Article 50 in 2016 (pictured, Ms Miller in London last week)

The case comes after former appeal judge Sir Richard Aikens wrote an article suggesting that the government may have acted illegally when Mrs May delayed 'exit day' without prior parliamentary approval.

Sir Richard wrote: 'The way the government has acted, by trying to force Parliament to agree to what it has done unilaterally without prior parliamentary approval, is, at the least, highly unsatisfactory. It must be arguable that the government has acted illegally.'

Following Sir Richard's piece, four Conservative MPs last week wrote to Theresa May suggesting delaying Brexit in the way she did was illegal.

Sir Bill Cash, who was among the four, wrote: 'We ask that you fully explain the actions taken by your Government and why you say that what it has done and is doing is legal.

'We are gravely concerned that you are unlawfully seeking to extend the UK's membership of the EU.'

Tory MP Sir Bill Cash is among those who have suggested the delay may have been illegal

Despite the claims, Thom Brooks, Professor of Law and Government at Durham University Law School, said he 'doubted' the latest legal case would succeed.

'I think the idea is that Parliament voted to leave on 29 March, this date has passed without a new Act of Parliament and so we're out,' he told MailOnline.

'But Parliament did agree for the PM to have more time and both the UK and EU have agreed our 'deal' remains, or at least for now. This is through temporary extensions.

'Someone was bound to try this - but I'm not expecting a Miller-like result.'

The legal argument was posted online next to a donation button for the English Democrats but Mr Tilbrook insisted the High Court battle was 'not an electioneering position'.

He said: 'What we're trying to do is we're trying to raise money, obviously bringing a case like this is enormously expensive and we're trying to raise money for the case.

'We asked people to help us to bring a case, we have brought a case and we're going to pursuit it.

'The whole point of our case is that we're out. This isn't an electioneering position, this is a legal case. We're all pretty confident that our legal case is a strong one.'

If the case is successful then the UK will be outside the EU on WTO trading terms, Mr Tilbrook said.

He went on: 'I don't think from a standing start people on the Leave side were arguing for leaving on WTO terms.

'What we thought we would get if we negotiated properly was some sort of arrangement and agreement with the EU.

'But the negotiations have been conducted in a way that I'd certainly hope any provincial solicitor would have done a better job of it.'

Gina Miller launched her high-profile legal challenge in 2016, saying that Parliament had to approve the triggering of Article 50.

The High Court and later the Supreme Court both found in her favour, forcing MPs to vote on the formal start of the Brexit process.

Ms Miller has since become a prominent advocate of a second referendum on Brexit.