Boris Johnson will be using the Queen “to make a Conservative Party political broadcast” if he launches a new session of parliament just weeks before a general election, a constitutional expert is warning.

The prime minister is being urged not to “further abuse her position” – after the embarrassment of the Queen’s signature being used to shut down parliament before being declared unlawful by the Supreme Court.

Mr Johnson’s new plan is to prorogue parliament for just a few days, to allow a Queen’s Speech on 14 October when the monarch will set out his flagship domestic legislation amid huge pomp and ceremony.

Now the constitution unit at University College London says it is wrong to see this shorter shutdown as uncontroversial – because it will pave the way for a likely winter election.

“The Queen’s Speech will be not so much the government announcing the legislative programme for the next session, but more of an election manifesto,” warned Professor Robert Hazell.

“The Queen will have been used to make a Conservative Party political broadcast.”

Professor Hazell added: “It would bring more embarrassment to the Queen, dragging her again into political controversy.

“Boris Johnson has already caused the greatest constitutional controversy of her reign; he should not further abuse her position.”

The prime minister is poised to ask Buckingham Palace to prorogue parliament on Tuesday, skipping three days of planned business before the new session opens the following Monday.

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It had been thought the “normal prorogation” would avoid the controversy of the unprecedented five-week shutdown that ended in humiliation in the Supreme Court.

“I want to deliver on the people’s priorities,” the prime minister said, seeking to explain his motive.

“Through a Queen's Speech, the government will set out its plans for the NHS, schools, tackling crime, investing in infrastructure and building a strong economy.”

Professor Hazell acknowledged that the Queen’s Speech has “long been used by governments for propaganda purposes”.

But he added: “The speech has always been followed by a legislative session in which those bills were introduced.

“This would be the first Queen’s Speech when the government had no real intention of introducing the bills it had just announced, because it hoped that, within weeks, parliament would be dissolved for an early election.”