George Soros’ Open Society Foundation has donated £400,000 to one of the UK’s leading campaigns working to reverse Brexit.

The US billionaire donated the cash to the Best for Britain campaign, which advocates staying in the European Union rather than a soft Brexit, sources told the Guardian.

The organisation was co-founded Gina Miller, who took the government to court over its decision to invoke the Article 50 treaty terms to formally begin the process of leaving the EU.

It is chaired by former UK government minister and deputy UN secretary general Lord Malloch-Brown, who told the paper: ‘We, like millions of people, believe that Britain should lead, not leave, Europe.

‘We work with campaigners, businesses, unions, politicians and community groups to make sure everyone has a strong campaigning voice.

‘George Soros’s foundations have, along with a number of other major donors, also made significant contributions to our work. Indeed, through his foundations he has contributed £400,000.

‘But our small donors have collectively contributed more and the commitments of the other major donors also exceed this amount. So he is an important and valued donor but his funding is one among many sources.’

The organisation said that it had raised more than £413,000 in grassroots donations.

Soros remains best known in the UK for his bet against sterling which led to Black Wednesday, as the government was forced to abandon its peg against European currencies.