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The tax authorities will need up to £450m in extra funding and up to 5,000 extra staff to deal with the impact of Britain leaving the European Union without a deal, MPs have been warned.

Jon Thompson, the senior civil servant in HM Revenue and Customs (HMRC), also told the public accounts committee that he could not guarantee that a new customs system would be ready for Brexit in March 2019.

At a public accounts committee meeting, Thompson said the tax authority has already been given £78m from the £250m fund set aside by the government this year to plan for the possibility of the UK crashing out of the EU.

Most of that is being spent on dealing with customs but cash has also been used to plan for the impact no deal would have on indirect taxes, the welfare state and data sharing, MPs were told.

Thompson said the funding was enough for now but warned he was likely to ask for “significantly more” next year.

He said an extra 3,000-5,000 extra staff would need to be recruited to cope with a no deal outcome. He told the committee:

It will be several hundred million pounds if we are implementing the option of the United Kingdom leaving the European Union with no ongoing special relationship in April 2019. That is the most extreme version, I think, of leaving the European Union. In that scenario you are looking at an estimate of between £300-450m.

The committee was told that a new customs system, called CDS, which has been developed by HMRC for several years will be ready for implementation by January 2019 - just two months before Brexit.

Labour’s Shabana Mahmood was among several MPs who questioned if this gave the government enough leeway. “If this goes wrong it will be catastrophic for Britain’s international reputation,” she said.

Thompson said he could not guarantee that a new customs system would be delivered on time, but that he hoped it would be. He said:

