The UK's chief taxman received death threats after revealing it would cost £20bn to put in place the hard-Brexiters' customs plan, he revealed today.
Jon Thompson, chief executive of HMRC, told MPs the figure in May as he gave evidence on a range of post-Brexit systems.
The system, dubbed maximum facilitation, or 'max fac', would see the UK operate a different customs regime to the EU and rely on technology to speed up checks on goods at the border.
Speaking at an Institute fo Government event on Thursday, Thompson revealed the disclosure of the cost of the scheme had led to "very significant personal consequences."
He said: "We have had to literally change how I travel and what my personal security is. We have had two death threats investigated by the Metropolitan Police for speaking truth unto power about Brexit.
"Those are real situations. I’m still not going to back away from [telling the truth] if I think something’s not going to work – it’s incumbent on me.
"We live in a democracy so in the end, it’s for governments to decide, ministers to decide what they want to implement.
"But our role as civil servants is to act with integrity and to give them our best advice."
Anti-Brexit Labour MP Alison McGovern, a member of the Treasury select committee, was appalled by the threats to Thompson.
She said: "This can't go on. We have got to be able to talk about this issue properly with out resorting to this kind of horrible behaviour."
Fellow committee member Simon Clarke, a Tory MP who backs Brexit and supports the very system Thompson was costing, described the threats as "abhorrent".
"It's desperately shocking that public servant doing his duty and appearing before a select committee to offer evidence to Parliament should be treated in this way," he said.