Mark Zuckerberg has come under intense criticism from the UK parliamentary committee investigating fake news after the head of Facebook refused an invitation to appear before them.

Prime Minister Theresa May even weighed in on the issue, urging the social media mogul to co-operate with the inquiry.

The social media billionaire's decision to instead send a senior executive to give evidence to the Commons Digital, Culture, Media and Sport Committee, was branded "absolutely astonishing" by its chairman.

The disclosure came amid a report by CNN that Zuckerberg had agreed to testify to Congress in the United States over the release of millions of users' data details to the UK firm, Cambridge Analytica.

Appearing before the Commons Liaison Committee on Tuesday, Mrs May said: "Mr Zuckerberg will decide for himself whether he wants to come before the committee but what I hope is that Facebook will recognise why this is so significant to people, why people are so concerned about it and ensure that the committee is able to get to the answers that they want."

Earlier Digital, Culture, Media and Sport Committee chairman Damian Collins insisted Zuckerberg himself was "the right person to give evidence" and repeated his call for him to face MPs either in person or by video link.

Speaking after the committee took three and a half hours of testimony from whistle-blower Christopher Wylie, Mr Collins said: "Given the extraordinary evidence we have heard so far today... I think it is absolutely astonishing that Mark Zuckerberg is not prepared to submit himself for questioning in front of a parliamentary or congressional hearing, given that these are questions of fundamental importance to Facebook users as well as to our inquiry.

"I would certainly urge him to think again if he has any concern for the people who use his company's services."

Mr Wylie is a former employee of Cambridge Analytica (CA), which allegedly used data gathered from Facebook users in its targeted messaging on behalf of Donald Trump's election campaign in the US.

He said he believed the closely linked Canadian company AggregateIQ had played a "very significant" role in securing a Leave vote in Britain's 2016 referendum on EU membership.

AggregateIQ (AIQ) took 40% of the spending by the Vote Leave campaign to target messages at a "very specific cohort" of voters who they believed could swing the result, he said.

And he claimed that a representative of the company, which also did work for the pro-Brexit BeLeave, DUP and Veterans for Britain campaigns, told him that its activities were "totally illegal".

Vote Leave campaign director Dominic Cummings dismissed Mr Wylie as a "fantasist-charlatan" in a blog post written while the pink-haired whistle-blower was still giving evidence.

By the time inquiries by the Electoral Commission and Information Commissioner's Office were complete, "the Observer will look really silly for making a hero out of a fantasist-charlatan, we'll already have left the EU, and Zoolander will need a new look," predicted Mr Cummings.

In a letter to Mr Collins, Facebook's head of public policy Rebecca Stimson said that the company would be putting forward chief technology officer Mike Schroepfer or chief product officer Chris Cox to answer MPs' questions.

Both are among Facebook's longest serving senior representatives and are "well placed to answer the committee's questions on these complex subjects" straight after the end of Parliament's Easter recess on April 16, she said.

Mr Collins said the committee would be happy to issue an invitation to Mr Cox, but made clear that he was not withdrawing the call for Mr Zuckerberg to give evidence.

"Facebook has got many questions to answer that their executives have failed to answer in previous appearances before our committee," he said.

"Given the seriousness of these issues, we still believe that Mark Zuckerberg is the right person to give evidence... He stated in interviews that if he is the right person to appear, he will appear. We think he is the right person and look forward to hearing from him."

Mr Zuckerberg's refusal to appear was branded "cowardly (and) completely unacceptable" by Labour deputy leader Tom Watson.