Facebook was the decisive factor in the Brexit vote and in putting Donald Trump in the White House, leaders of those campaigns have said.

The US social media giant may now be just as pivotal in the general election in Britain where almost half the population has an account.

The claims are made in a BBC1 Panorama documentary laying bare how political campaigns used Facebook to pinpoint voters, reports the Daily Mail.

Leaders of the campaigns for Brexit and for Trump spent tens of millions of dollars with Facebook. It has nearly 2 billion users worldwide, many of whom simply share photographs and keep in touch with friends.

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However, Facebook collects data about them based on every conversation they have, every post they 'like' and every picture or post they upload.

It then uses this information to ensure advertisers - and political parties - get their messages to precisely the people they want.

Gary Coby, director of advertising for President Trump's Republican Party, said: "We take the name, address, phone number, email and we can take those pieces of data and send it on to Facebook.

"Facebook then matches them to the user's profile.

"So if you are on Facebook, I can match you and put you into a bucket of users that I can then target."

Critics fear Facebook has too much power. Photo / AP
Critics fear Facebook has too much power. Photo / AP

Gerry Gunster, a political strategist from the Brexit campaign Leave.EU, said Facebook was key to its success.

"You can say to Facebook, I would like to make sure that I can micro-target that fisherman in certain parts of the UK so that they are specifically hearing that if you vote to leave that you will be able to change the way regulations are set for the fishing industry.

"I can do the same thing for people who live in the Midlands who are struggling because the factory has shut down. So I may send a specific message through Facebook to them that nobody else sees."

Political commentators have long thought Facebook was key to the Brexit vote and the US election.

A political strategist from the Brexit campaign Leave.EU said Facebook was key to its success. Photo / File
A political strategist from the Brexit campaign Leave.EU said Facebook was key to its success. Photo / File

But this is the first time those who led the campaigns have said so.

Coby admitted the official Trump campaign spent around $101 million on Facebook in the run-up to the US election, helping it to target specific groups in swing states.

"The way we bought media on Facebook was like no one else in politics has ever done," he said.

Critics fear the social media group, whose huge advertising operation helps it bring in $1.45 billion in profits every month, has too much power.

British Conservative MP Damian Collins, who chaired the culture, media and sport committee before Parliament was dissolved, said Facebook is "totally unregulated" and should be more accountable.

Facebook said it complies with all the regulations it is obliged to.