Boris Johnson was in breach of the British constitution
The constitution does not allow a Prime Minister to prorogue parliament for the purpose of stymying debate and bypassing the democratic process.
Boris Johnson needed to provide the Queen with his reasons for granting his request for a prorogation of parliament. In giving these reasons, he lied and misled the Queen. Had she known the real reasons, she may not have granted his prorogation. Therefore, the prorogation is illegal, as it was secured on false grounds.
He told the Queen that he wanted to prorogue parliament to lay out his legislative agenda. In reality, his true motive was to block MPs from derailing his plans to pull Britain out of the EU on October 31. It is not also not customary for a new prime minister to hold a Queen’s speech, as Johnson claims. Theresa May never held one. The only time she ended parliament was during the general election in 2017. Under Gordon Brown, parliament was prorogued, and the Queen delivered her speech 125 days after he entered office. Boris Johnson was certainly under no pressure to hold a Queen’s speech before the October 31 Brexit deadline.
Boris Johnson Didn't Lie Boris Johnson did not lie to the queen. Brexit is not the only thing going on in Britain. There are other issues that need seeing to and the ending of the parliamentary session was an opportunity to give those other issues a platform and a place on the legislative agenda. As a new prime minister, Boris Johnson also needed to hold a Queen’s speech. 47 days had passed since he took office and he had still not held a speech. This was highly irregular and a break with leadership conventions. If He Did, It Wouldn't Make a Difference Even if he did lie to the Queen, that wouldn't make the prorogation unlawful. Firstly, none of the treason acts (1351, 1702, 1708, or 1848) declare that lying to the monarch is a treasonous act. Secondly, the Queen, at this stage, is merely a rubber stamp. She does not intervene in politics and would grant her prime minister a prorogation at his request, regardless of the reasons. Whatever Boris Johnson's motives for the prorogation, the very fact that she approved it makes it lawful. Finally, even if we accept that lying to the Queen about his motives for prorogation is illegal, it would also be on the court to prove that Johnson lied to the Queen. Given that the conversation only took place between Boris Johnson and her majesty, and she isn't going to testify, we will never know whether he lied or not.
[P1] Securing permission from the Queen on false grounds is illegal. [P2] The motives Johnson gave to the Queen for his decision to prorogue parliament were not his true motives. [P3] Therefore, the prorogation is illegal.
[Rejecting P1] Lying to the Queen is not illegal in any form or capacity. [Rejecting P2] Johnson did not lie to the Queen.