Boris Johnson and Michael Gove have attacked HM Revenue and Customs on behalf of the “plucky individuals” who bankrolled Brexit and now face huge tax bills.

A source close to Mr Johnson said the demands were “bad for our democracy” while a friend of Mr Gove said they were “an attempt to silence anyone who dare challenge the Establishment and status quo”.

Their comments came as Peter Cruddas, a City financier and one of the Leave donors, disclosed that he had given hundreds of thousands of pounds to other political campaigns – including the 2011 Alternative Vote referendum – and never been challenged by HMRC.

A senior Leave-supporting Conservative politician described the demands as “scandalous” and evidence of “the Treasury’s revenge” for losing the European Union referendum.

The Telegraph disclosed on Saturday that inheritance tax demands running into millions of pounds had been sent this month to at least three major donors to the campaign to leave the EU – Mr Cruddas, Lord Edmiston, a Midlands entrepreneur, and Arron Banks, an insurance tycoon, as well as to a major Remain supporter, David Harding.

The demands, which force people to pay the 20 per cent tax up front on large “gifts”, will disproportionately hit Leave-supporting donors because the various “out” campaigns were financed by entrepreneurs rather than mainstream publicly listed companies that tended to back Remain.