William Hague has warned that Brexit would be a big boost for the UK's opponents, who would love to "cause mischief" in the event of Leave vote.
"Away from the cameras on Friday morning – if by then we have voted to leave – President Putin would allow himself a thin but satisfied smile," Hague wrote in the Telegraph.
"Working out how to play the EU with Britain departing from it would not take long: lie low for six months until sanctions on Russia can be lifted, and then feel free to put the West under pressure – in Ukraine, or Moldova or Georgia or wherever – once Britain is no longer a voice in Brussels for transatlantic unity when the Kremlin mounts its next adventure.
"A solid European approach to reducing dependence on Russian energy? Forget it. The reimposition of sanctions when Moscow misbehaves? Less likely. Instead, there would be the wonderful spectacle of the major Western European countries consumed with renegotiating their arrangements with each other. Certainly enough reason for an extra round of vodka on Friday night."
The former cabinet minister went on to say state that the UK passed a law in 2011 that requires a referendum of the British people if any new powers or competences are ever to be transferred to the EU.
"The idea that we could be forced to rely for our defence on a European Army is therefore a fiction. And support on the continent for tighter integration is evaporating," Hague continued.
"For [the SNP], I have long argued, a result in which Scotland votes to remain in the EU but England tips the whole of the UK into leaving, would be a great gift. This has been one of the neglected subjects of this campaign – understated by the Remain side because the SNP are part of it, and ignored by the Leave side because it is a highly inconvenient truth," he wrote.
"The majority wishes of the Scottish people have been ignored, they will say, and they are now being removed from the EU against their will in a travesty of democracy."