The taxman would need up to £450 million in extra funding to deal with the impact of Britain quitting the European Union without a deal, MPs have been warned.
HM Revenue and Customs (HMRC) has already been given £78 million from the £250 million pot of funds the Government has set aside this year to plan for the possibility of the UK crashing out of the bloc.
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, the Public Accounts Committee was told.
HMRC's top civil servant said the funding was enough for now but warned he was likely to ask for "significantly more" next year.
Jon Thompson 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-£450 million."