Investigation into Brexit backer Arron Banks comes amid deepening uncertainty over future of the UK-EU ties.

British officials are investigating a prominent Brexit backer and his influential campaign group over alleged financial misconduct linked to the United Kingdom’s 2016 referendum on leaving the European Union.

In a statement released on Thursday, the UK’s National Crime Agency (NCA) said it has initiated a probe into Arron Banks and the Leave.EU campaign group, among others, over “suspected electoral law offences … as well as any associated offences”.

“While electoral law offences would not routinely fall within the NCA’s remit, the nature of the necessary inquiries and the potential for offences to have been committed other than under electoral law lead us to consider an NCA investigation appropriate in this instance,” the statement said.

The NCA, which is tasked with investigating serious and organised crimes, said it acted following a referral of material from the UK’s Electoral Commission.

The investigation is sensitive, because it could potentially cast doubts on the legitimacy of the Brexit vote.

In a statement also released on Thursday, the Electoral Commission alleged Banks was “not the true source” of eight million British pounds ($10.4m) of loans provided to Leave.EU, an unofficial Brexit campaign group, and Better for the Country Limited (BFTC), a company that was used to finance Leave.EU.

The statement marked the conclusion of an Electoral Commission inquiry into payments related to the referendum made to the two groups, during which Leave.EU and BTFC representatives told investigators that $10.4m of loans were financed by Banks.

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The commission said it found “reasonable grounds to suspect a number of criminal offences” were committed in relation to the payments, including the involvement of an “impermissible company” and deliberate concealment of financial transactions.

Under UK law, loans and donations to registered political campaigners can only come from so-called permissible sources, essentially excluding foreign funding.

In July, a UK parliamentary committee said it was “unclear” where Banks had obtained the money he donated to the Leave campaign and whether he had “profited from business deals arising from meetings arranged by Russian officials”.

On Thursday, Banks claimed there was “no evidence of any wrongdoing” by the companies he owns and denied he had been the recipient of any donations from “foreign” sources.

“I am confident that a full and frank investigation will finally put an end to the ludicrous allegations levelled against me and my colleagues,” he said in a statement published on the Leave.EU website on Thursday.

“I am a UK taxpayer and I have never received any foreign donations … The Electoral Commission has referred me to the National Crime Agency under intense political pressure from anti-Brexit supporters,” he added.

‘We remain vigilant’

The UK is due to leave the European Union on March 29, 2019, nearly three years after 52 percent of Britons voted in favour of ending the country’s 43-year membership of the 28-member bloc.

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But some Britons who favoured remaining with the EU have called for a rerun of the vote, alleging the campaign to leave the bloc was marred by financial improprieties and possible foreign – or even Russian – funding for the campaign.

On Thursday, a spokesperson for British Prime Minister Theresa May said the government had “not seen successful interference in UK democratic processes”.

“We of course remain vigilant and we will continue to work to strengthen our democracy against potential interference,” the spokesperson added.

‘Brexit must be put on hold’

Opposition Labour Party leader Jeremy Corbyn made no initial comment on the NCA’s announcement but a number of Labour parliamentarians were quick to call for a suspension of the Brexit process in light of the NCA’s new probe.

MP David Lammy said Thursday’s developments meant “Brexit must be put on hold until we know the extent of these crimes against our democracy”.

“We already know the electoral law was broken. Now Aaron Banks is under investigation because Leave. EU stands accused of spending foreign money,” Lammy said on Twitter.

We already know electoral law was broken. Now Aaron Banks is under investigation because https://t.co/pbNTdJCphb stands accused of spending foreign money. Brexit must be put on hold until we know the extent of these crimes against our democracy.https://t.co/WrJcXng8o7 — David Lammy (@DavidLammy) November 1, 2018

Lammy’s comments were echoed by fellow Labour MP, Helen Hayes.

“The government cannot & must not continue blindly towards the constitutional & economic precipice of Brexit while such serious allegations of the undermining of our democracy remain,” Hayes said on Twitter.

Negotiations over a possible Brexit divorce deal between the UK and the EU have hit a stumbling block in recent weeks, mainly due to unresolved concerns over how the border between Ireland – an EU member state – and Northern Ireland – which is part of the UK – will be managed once the UK leaves.