Former deputy prime minister Nick Clegg has predicted that international trade secretary Liam Fox will resign "in a huff" within the next 18 months.
The ex-Liberal Democrat leader said Fox headed a department "without a purpose" while Britain remained in the European Union's customs union.
He said Fox was a "proud man" and would eventually quit the cabinet in frustration at the slow pace of Brexit, to join "the angry brigade on the right wing".
Clegg, MP for Sheffield Hallam, was speaking to journalists in Westminster on Tuesday. He declined to answer a question about what former prime minister David Cameron really thought of Fox.
"I’m not a betting man, but if I was I would put a fair amount of money that Liam Fox will resign in a huff in the next 18 months," Clegg said. "I’ll explain to you why – he doesn’t have a job and he doesn’t appear to have realised that yet.
"He genuinely doesn’t have a job. If the UK doesn’t leave the customs union – which apparently is still an open question in Whitehall – then he’s heading a department without purpose, because he cannot negotiate all these apparently magnificent trade deals with China and India and Australia.
"It’s only 15% of British trade goes to countries which are outside the EU or with whom we don’t have an EU trade deal ... So, poor chap, if they decide – and it won’t be his decision – not to quit the customs union, it really will be a little embarrassing, because his department will have nothing to do."
The Financial Times reported in July that Fox is pressing prime minister Theresa May to pull out of the EU customs union, an area that allows the free movement of goods with no tariffs. If it doesn't leave, Britain will be forced to keep the same external tariffs as the customs union in trade deals around the world.
Clegg said: "He’s a proud man and at the same time he’ll have lots of his colleagues on the Conservative back benches who will start, as they always do on the right of the Conservative party, developing this betrayal myth.
"It’ll always be someone else’s fault that Brexit hasn’t happened by next Tuesday. It’ll be the civil servants, the foreigners, the City…
"They’ll get increasingly agitated and at one point Liam Fox will have to ask himself, 'Am I going to sit here and become an increasingly laughing stock because I have nothing to do, or do I join my spiritual fellow travellers, the angry brigade on the right wing, and say I was also betrayed?' He’ll do that."
Clegg said that even if Britain does pull out of the customs union, Fox "still hasn't got anything to do". He pointed to Australia, which he claimed had made it "quite clear they have absolutely no intention of signing a trade deal" until the long, complex process of Brexit had been completed.