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The UK would find itself in a "worse position" than it is now, he said, if it left the existing arrangement but negotiated a similar new one.
Having to accept EU rules and limits on doing other deals would make the UK "less attractive", he said.
But his former top official has criticised the government's strategy.
Sir Martin Donnelly, who was permanent secretary in the Department for International Trade until last year, said any deals done after Brexit would not compensate for leaving the single market and the customs union.
Giving up access to the EU market and its existing trade agreements was "rather like rejecting a three course meal now in favour of the promise of a packet of crisps later", he said.
Sir Martin, who has previously warned about leaving the single market and has worked for the European Commission in the past, said that negotiating full access to the single market without accepting EU rules would require a "fairy godmother specialised in trade law".
Foreign Secretary Boris Johnson said he did "not agree for a moment" with Sir Martin's verdict, because the "real growth opportunities" were outside the EU.
Mr Johnson told BBC Radio 4's Today programme that a long-term customs union, as Labour has proposed, would lead to "colony status for the UK", which would not get a say in trade policy. [...]