UK ministers, including the Prime Minister David Cameron, plan to travel regularly to Scotland to push the message that “Scotland has two governments”, each looking out for the nation’s interests, and that the SNP is not the country’s sole voice.

The visits will also be used to press Salmond on the timing and format of the referendum, and to claim that uncertainty is deterring investment.

Alistair Carmichael, chief whip for the Liberal Democrats, said of the referendum: “Bring it on.”

Carmichael claimed Salmond was being deliberately vague because he was too scared to talk about the detail of independence, as that would expose the SNP’s confusion on the practicalities and alarm voters.

But the SNP said it would pay the bus fares for Tory and LibDem ministers, “because every visit is worth a thousand votes to the SNP”.