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The International Trade Secretary, Liam Fox, will say in a speech later that the UK's continued membership of a customs union after Brexit would be a "complete sell-out" of Britain's national interests.

But Sir Martin Donnelly, formerly the most senior civil servant in Dr Fox's department, said that ending customs union arrangements would be so damaging, the UK would have to ask to rejoin the single market.

"Brexit is like giving up a three-course meal... for the promise of a packet of crisps in the future," he said.

(Image: Sir Martin Donnelly, credit: Getty