Hedge-fund billionaire and Donald Trump backer 'played key role in Brexit campaign'

A US hedge-fund billionaire who helped finance Donald Trump's campaign to become US president reportedly played a key role in the campaign for Britain to leave the EU.

Robert Mercer, co-owner of right-wing news organisation Breitbart, allegedly directed his data-analytics firm Cambridge Analytica to provide expert advice to the Leave campaign.

Mr Mercer, whose firm was paid £4.8m by the Trump campaign to persuade swing voters, offered his firm's help to Ukip leader Nigel Farage for free, Leave.eu communications director Andy Wigmore told The Observer.

“They were happy to help. Because Nigel is a good friend of the Mercers," Mr Wigmore said.

"What they were trying to do in the US and what we were trying to do had massive parallels. We shared a lot of information.”

The firm is said to have advised Leave.eu by harvesting data from people's Facebook profiles to decide how to target them with individualised advertisements.

However, the electoral commission was not informed of Mr Mercer's work. All services worth more than £7,500 must be declared.

Leave.eu declined to tell the paper why it had not declared the donation of services.

Leave.eu founder Arron Banks previously told The Observer: "AI won it for leave."