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The UK's Customs chief has had to report two death threats to the police for speaking 'truth to power' on Brexit.

Jon Thompson, who is head of the HMRC, received a furious backlash after telling a Parliamentary Select Committee the potential cost to businesses of post-Brexit customs options.

The tax chief revealed that the death threats had forced him to change how he travelled and his security plans.

He told MPs in May that the “max fac” system favoured by Brexiteers could cost business between £17bn and £20bn - or £385million a week.

So-called "max fac", would use technology to prevent a hard border on the island of Ireland, is preferred by leading Brexiteers.

Speaking at an Institute for Government event today, the tax chief, 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,” he said.

Asked about the impact of the death threats by Civil Service world, he said: “You know you’re in a [significant] moment because the question is a very powerful one and the answer is very stark,” he said.

“The first I knew it was significant was when my 28-year-old son text me with, ‘you’re trending on Twitter’. [I thought] 'oh, is that a good thing? I don’t know'."

(Image: REUTERS/Darren Staples)

He went on to say how important it was that the Civil Service give government the best advice regardless of how it might be received.

“I didn’t realise it would result in that, but I think it is absolutely incumbent on us to stick to the fundamental principles of the civil service, which is to give ministers the best advice that we can, and in a democracy minsters make the decisions.

“If what we are going to do is back away from that, for whatever reason, I don’t think that’s right. For me that is about personal integrity, and sometimes it is really really difficult – and it is tough with ministers – but it is the right thing to do.”

He added that the fact he had revealed the £20bn figure provided a boost to staff in HMRC.

“Colleagues at HMRC sit round and watch if I’m at a select committee and they give me feedback, which I want to encourage them to do because it is that kind of organisation, but I didn’t realise it had so much resonance in the organisation – that there’s the boss telling it as it is, we know that is true, we have that information and ministers have that information too.”

A number 10 spokeswoman said the threats were "very concerning" and "completely unacceptable".