The taxman paid Facebook to warn others to settle their bills on time, despite a public outcry over size of the tech giants' own payments.

HM Revenue and Customs (HMRC) has admitted that it advertises with the social media site.

Facebook hit the headlines last year when it emerged that it paid just over £4,000 in corporation tax in the UK in 2014.

It is understood to be currently challenging an audit into its UK operations.

Reports yesterday also suggested that while the company has set aside more than £2 billion to settle global tax disputes, none of the money is destined for UK coffers.

There is growing pressure on global tech firms over the amount of tax that they pay in the UK.

A major row erupted last week when it emerged that Google had reached a settlement with HMRC to pay just £130 million in back taxes stretching over 10 years.

A spokesman for HMRC said that figures on how much it spent on Facebook advertising were not "readily available".

He added: “You can try an FOI (Freedom of Information) but I'm told it would likely break the cost exemption.”

Government bodies can refuse FoI requests on the ground that the costs involved would be prohibitive.

Last night a spokesman for Facebook said: “We are compliant with UK tax law and in fact all countries where we have employees and offices."

But Scots Tory leader Ruth Davidson took issue with that attitude – as she called on her party’s government at Westminster to do more.

She said: We need a tax system which is competitive and business-friendly but which is also seen as fair...

"When asked about their affairs, firms like Google and Apple have the same reply – they have paid exactly what HMRC has asked them to pay and have complied with the law.

"And they are right.

"Then, when asked about the government's response, David Cameron and George Osborne point out that no government has done more than this to crack down on tax evasion and aggressive tax avoidance.

"And they are right too.

"The trouble is, both might be right, but it doesn't feel right. It doesn't feel fair. And in our hearts, I think we all know it isn't fair.

"How can it be, when families across the UK see money taken off their pay cheque every month before it even reaches them – only to see these fabulously wealthy multinationals able to dictate the sums they pay?”

She warned against adding to the sense that the system is rigged against ordinary people.

She added: "The populists, the demagogues and the extremists – the Trumps and the Le Pens – ... are hoping to exploit that resentment for their own narrow purposes".

Today the SNP calls on Google to publish details of how much it has made in the UK since 2003.

MPs want to pour over the company's accounts before representatives from Google and HMRC are grilled by the House of Commons Public Accounts Committee (PAC) next week.

The SNP has also written to the European Commissioner for Competition, Margarethe Vestager, asking her to examine the Google settlement.

SNP MP Deirdre Brock, a member of PAC, said: “Google UK also has a duty to its customers across the UK to play a part in facilitating public scrutiny of their tax settlement.

"By publishing details of their revenue, they can demonstrate their commitment to open public scrutiny of the level of tax that they pay in this country.”