Contingency plans for customs post-Brexit should be "pushed forward", the UK's public spending watchdog boss, Sir Amyas Morse, has told MPs.
Revenue & Customs is installing a new system two months before Brexit in 2019, and it has said £7.3m is needed in case it does not work in time.
Sir Amyas told the Treasury Committee this was "not a very significant sum of money in the scheme of things".
"If I was giving you advice... I would say push that forward," he said.
The National Audit Office head said: "I would have thought having a contingency plan fully prepared would be something you wanted to crack on with straight away.
"There are things like that where I feel that it's not a good idea to operate acceleration and the brake at the same time."
Tory MP Charlie Elphicke said: "There are a whole load of other people saying we shouldn't spend money on contingency that might be wasted.
"To which I argue well actually you get house insurance before the house is burgled and much of this is spending we should have to have tip-top borders in any event."
It follows comments last week from HMRC boss Jon Thompson, who told the Public Accounts Committee he was "reasonably confident" the new system would be ready in January 2019.
Funding was nevertheless needed for a contingency plan, he added, and he was in discussions with the Treasury.
The move to the updated Customs Declaration Service (CDC) was planned before the UK voted to leave the EU, but the completion date was moved forward after the referendum result.
Mr Thompson said the extra £7.3m would be used to make sure the old system could cope with extra work generated by Brexit.
Sir Amyas said government departments were beginning to realise "the scale of the task" of Brexit.
He had previously criticised the government's "vague" Brexit plan, saying ministers had to be more "united" or the project would fall apart "at the first tap" like the segments of a "chocolate orange".