The Labour leader has written to Sir Mark Sedwill warning it would be an “anti-democratic abuse of power” if the prime minister was to deny voters a choice on Britain’s EU future in an election campaign.
The move comes amid reports Johnson could seek to hang on long enough to ensure Britain is out of the EU before going to the polls if he is defeated in a vote of confidence when MPs return in September.
As it stands - under the latest extension to the Article 50 withdrawal process by the EU - Britain is due to leave on October 31.
But with a wafer-thin Commons majority of just one, Johnson is vulnerable to defeat if, as expected, Labour table a no-confidence motion early next month.
If that happened, under the Fixed Term Parliaments Act, he would have 14 days to win another vote of confidence or, if no other government could be formed, face a general election.
His top adviser Dominic Cummings is said to have argued that would still allow him to set an election date after October 31, by which time the UK would be out of the EU with nothing a new government could do to stop it.
In his letter to Sedwill, Corbyn said such a course of action would be “unprecedented” and “unconstitutional”.
He said the Cabinet Office’s election “purdah” rules make it clear that policy decisions on which a new government “might be expected to want to take a different view” should be postponed until after polling day.
He asked Sir Mark to confirm that if the UK is due to leave the EU without a deal while an election is under way, the government should seek another time-limited extension to Article 50 to allow the voters to decide.
“Forcing through no-deal against a decision of parliament, and denying the choice to the voters in a general election already under way, would be an unprecedented, unconstitutional and anti-democratic abuse of power by a Prime Minister elected not by the public but by a small number of unrepresentative Conservative Party members,” he wrote.
“A Labour government will never support a no-deal exit, so would of course ‘want the opportunity to take a different view’.”
But Brexiteers argue parliament has already voted to leave the EU, voting to trigger the Article 50 process and passing legislation setting Britain’s withdrawal date for October 31.