John Major to challenge Boris Johnson's decision to suspend Parliament at Supreme Court
Sir John Major is to publicly argue that Boris Johnson acted unlawfully by suspending Parliament.
The former Prime Minister is due to speak for 20 minutes at the Supreme Court, which is hearing two appeals over Mr Johnson's controversial decision.
The court hearing will begin on Tuesday and could run until Thursday, meaning the judgement may not be released until next week.
At the Court of Session in Edinburgh last week, judges ruled that the PM had effectively broken the law by recommending to the Queen that Parliament be prorogued until 14 October.
Ministers have insisted it was a routine move ahead of a Queen's Speech on that date setting out the Government's legislative agenda.
But the Scottish judges said the true motive was to "stymie" MPs' opportunity to debate Brexit in the run-up to the UK's scheduled departure from the EU on 31 October.
The Supreme Court will hear the Government's appeal against the ruling, as well as an appeal on a separate case by anti-Brexit campaigner Gina Miller, backed by Sir John Major.
At the High Court in London last week, judges rejected Ms Miller's case, insisting that the row was a matter for politicians to consider rather than lawyers.
Speaking last month, Sir John said: "I intend to seek to assist the Court from the perspective of having served in Government as a Minister and Prime Minister, and also in Parliament for many years as a Member of the House of Commons.”
However, critics have pointed out that Sir John himself prorogued Parliament earlier than necessary ahead of the 1997 general election, thereby dodging a Commons debate on the cash for questions controversy.
Meanwhile, Boris Johnson has told the BBC he will "wait and see what the judges say" before deciding to recall Parliament early.
Downing Street officials have already stated that MPs will return to the Commons if the Supreme Court rules against the Government.