The High Court will rule as early as Christmas whether Brexit should be declared “void”, in a legal case given a turbo-boost by the criminal investigation into Leave funder Arron Banks.

Judges are poised to fast track the potentially explosive challenge, after Theresa May’s refusal to act on the growing evidence of illegality in the 2016 referendum campaign, The Independent can reveal.

Lawyers describe that failure as “absolutely extraordinary” – given the National Crime Agency’s (NCA) probe into suspicions of “multiple” criminal offences committed by Mr Banks and the Leave.EU campaign.

Now The Independent understands the case is likely to move to a full hearing and a ruling within weeks of opening on 7 December, with the clock ticking on the UK’s departure from the EU next March.

Both its lawyers and a leading academic believe its chances of success have been given a big boost by the unfolding scandal and the government’s refusal to recognise the gravity of what is being exposed.

The government is expected to deploy Sir James Eadie QC – the star barrister who led the unsuccessful battle for the government to trigger Article 50 without parliament’s consent – in a sign of the case’s importance.

Ewan McGaughey, a senior law lecturer at King’s College London, has written: “The order by the prime minister to trigger Article 50 and negotiate to leave the EU could be declared void.

“Now, it’s a big thing to litigate the very validity of Brexit. But if Russian athletes win Olympic medals when they are taking drugs, their victories are not valid. The same is true of a corrupt vote.”

One senior Tory told The Independent: “If Arron Banks were to be charged and found guilty after the police investigation, he could go to prison. The man who played a big role in securing Brexit could yet help to destroy it.”

And Ben Bradshaw, a Labour MP who has pressed for a full investigation into Mr Banks’ activities, said: “This case should be explored given the allegations and the very serious crimes we know were committed by the Leave campaign.”

Sue Wilson, who has lived in Spain with her husband for 11 years and helped bring the case, said: “We have heard far too often that Brexit is the ‘will of the people’.

“On the evidence now available, it cannot be said with any degree of certainty that the result would have remained the same had the law not been broken.”

The case will argue that Brexit must be declared void and the notification of Article 50 quashed, because “various criminal offences may have been committed”.

Mr Banks could be given a two-year prison sentence if he is found to have “knowingly concealed” that an Isle of Man company was the source of up to £8m for Leave.EU – which would have been outlawed as a foreign donation. He denies the charges.

Meanwhile, spending by both Leave.EU and the main Vote Leave campaign, fronted by Boris Johnson and Michael Gove, was found by the Electoral Commission to have broken the law.

But Downing Street has fought off calls for a “Mueller-style” inquiry, on the scale of the US probe into alleged collusion with Russia by Donald Trump’s presidential campaign.

Instead, Ms May has repeatedly downplayed the commission’s findings – even though they were to a “criminal standard of proof” – and has hailed the referendum as “a great exercise in democracy”.

Croft Solicitors wrote on 6 November, urging the prime minister to “reconsider her refusal to take any action”, following the announcement of the NCA investigation.

It urged her to order “an independent and speedy investigation” into whether the referendum campaign was lawfully conducted and “consider how best to conduct another referendum”.

The government was urged to reply by 14 November, but has yet to do so – with the provisional hearing less than two weeks away.

It had been expected that any ruling would await the conclusions of the NCA probe, but it is now thought it will be fast tracked – with just four months until Brexit day.

Rupert Croft, who will represent the British expats in other EU countries, said: “My clients think that it’s absolutely extraordinary that the prime minister refuses to take any action whatsoever in relation to the information uncovered about the conduct of the EU referendum that casts doubt upon its legitimacy.

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