Nicola Sturgeon has reacted to suggestions that Mr Fox, the Trade Secretary, favours such a move tweeting that it would be a “terrible decision”.
Her SNP colleague Joan McAlpine has argued that it would harm Scottish businesses’ ability to trade.
Dr Fox wants maximum freedom to negotiate new global trade deals and is determined that the UK should break out of the EU framework,
But the SNP said exiting the customs union would jeopardise the free movement of goods.
Ms McAlpine said: “Pulling the UK out of the customs union would be a deeply damaging step – adding costs and inhibiting the ability of Scots businesses to trade.”