Boris Johnson’s plan to prorogue parliament for five weeks is the longest suspension of the British legislature since 1945 and appears designed to evade scrutiny of his plans for Brexit or any new deal with the EU, constitutional experts have said.

Ruth Fox, the director of the highly respected Hansard Society, a leading source of independent research and advice on parliamentary affairs, said it was an “affront to parliamentary democracy”.

The Commons Speaker, John Bercow, has said the move was an “offence against the democratic process” designed to stop MPs debating Brexit.

Lawyers said the unprecedented length of the suspension of parliament would prompt immediate legal challenge. “Expect litigation – starting in Scotland,” said Ros Kellaway, the head of Brexit and EU law at Eversheds Sutherland, in London.

The prorogation could also have consequences for Northern Ireland, where a no-deal Brexit could increase the threat of direct rule from Westminster.

The lengthy gap between the end of the last scheduled parliamentary session, on 9 September, and the next sitting, on 14 October, compares with a “three to seven day gap” in the last decade, said Fox.

“The decision to prorogue parliament is not unconstitutional but the length of it is unprecedented in recent times. If the government wants a new Queen’s speech it could bring one forward on the usual week to two-week timetable for prorogation.

“In the current Brexit context, anything longer than this is clearly designed to help the government evade parliamentary scrutiny.”

Q&A What does 'prorogue parliament' mean? Show Hide Prorogation is the official term that marks the end of a parliamentary session. After being advised to do so by the prime minister, the Queen formally prorogues parliament. This takes the form of an announcement in the House of Lords on the Queen’s behalf. It is a speech, written by the government, which usually describes the bills that have been passed during that session and summarises what has been achieved. It means that all work on existing legislation stops, and MPs and Lords stop sitting. Prorogation also automatically kills any bills, early day motions or questions to ministers going through parliament. Parliament can then be reopened a few days later with a fresh slate of legislation intentions, set out in a new Queen’s speech at the formal state opening of parliament.

Meg Russell, a senior fellow of the UK in a Changing Europe initiative and the director of the constitution unit at University College of London, said: “What he’s doing, frankly, is hiding from parliamentary scrutiny: denying MPs the opportunity to question him and to hold him to account, and potentially denying them the opportunity to express no confidence in him.”

She continued: “He’s daring MPs to vote him out of office.

“If our constitution has one overriding principle, it’s parliamentary sovereignty: the government only is the government because it has the support of parliament.

“Boris Johnson was prime minister for not much more than 24 hours when parliament broke for the summer recess. He has faced no parliamentary scrutiny for the entire period since then.

“We’re looking at an unprecedentedly long prorogation, at a time when we are in the middle of the biggest political crisis since the second world war, and the prime minister has only been in office five minutes. It’s putting the Queen in a very difficult situation.”

Hansard said the last time there was a prorogation for longer than two weeks was in 2014, when parliament was suspended for 14 days. But this included time off for EU parliamentary elections and Whitsun recess.

Johnson’s supporters argue the five-week gap is needed to develop new legislation. Parliament is already in the longest-running session since 1945 because of the logjam created by Brexit.

Facebook Twitter Pinterest The longest parliamentary sessions since 1945. Photograph: The House of Commons Library and Hansard Society

However, Hansard says suspending parliament will deny democratic scrutiny as ministers will not have to account to parliament and select committees, which have been vital in investigating the consequences of Brexit.

“On their return in October MPs will have very little time to scrutinise any new deal with the EU, if there is one, or little time to scrutinise the remaining statutory instruments that the government may need for a no-deal Brexit,” said Fox.

Bercow, who has said he will “fight with every breath in my body” to stop Johnson from proroguing parliament to force through a no-deal Brexit without the consent of MPs, said: “However it is dressed up, it is blindingly obvious that the purpose of prorogation now would be to stop parliament debating Brexit and performing its duty in shaping a course for the country.

“Shutting down parliament would be an offence against the democratic process and the rights of parliamentarians as the people’s elected representatives.”

Catherine Haddon, a senior fellow at the Institute for Government, said the move did “seem like a very obvious move to try and cut down the time available to anti-no deal MPs to be able to do something about it”.