Boris Johnson should quit as Prime Minister if he has any "honour", the First Minister has said.

Nicola Sturgeon accused Johnson of attempting to "shut down democracy" and urged MPs to oust him in a vote of no confidence if he does not resign willingly.

It comes after the Supreme Court unanimously ruled he acted unlawfully in his bid to shut down parliament for five weeks.

The unprecedented legal verdict by the UK's highest court means the prorogation of parliament is "void", with MPs returning to the Commons to resume business on Wednesday morning.

The Prime Minister has called the court's judgment "unusual" but has said he will respect it even while he disagrees with it.

Earlier, the First Minister tweeted a picture showing her cabinet watching the historic ruling from her residence of Bute House.

The Supreme Court said UK ministers' advice to the Queen to suspend parliament was illegal because it "had the effect of frustrating or preventing the ability of parliament" to fulfil its constitutional role.

Johnson claimed the prorogation was to give his government time to produce a new legislative agenda in a Queen's Speech, but opponents said it was to dodge parliamentary scrutiny of his Brexit plans.

Speaking to STV News, Sturgeon described the court's ruling as "the most significant court judgment any of us have seen in our lifetimes".

She continued: "Politicians, in moments like this, tend towards hyperbole but it is no exaggeration say this is genuinely unprecedented and extraordinary.

"This judgment has found the Prime Minister acted unlawfully in seeking to prorogue parliament, that he really did so to escape scrutiny and accountability, and that he did all of this with no good reason at a time of political and constitutional crisis for the country."

The UK is still legally scheduled to leave the EU on October 31, but earlier this month backbench MPs forced through legislation to delay Brexit further to avoid a no-deal outcome.

The First Minister said stopping a no-deal departure remains the SNP's top priority at Westminster, but she also called for a general election "sooner rather than later".

She said: "I don't say this lightly - I think any Prime Minister with honour would tender his resignation after the judgment this morning.

"The Prime Minister has been found to have acted unlawfully and clearly the effect of that was to shut down democracy.

"I don't believe it would be right or proper to continue with a Prime Minister in office who has been found to have acted in that way.

"If Boris Johnson opts not to resign, parliament should take that decision out of his hands and vote him and his government out of office."

The First Minister added: "The positive thing today, as compared to yesterday, is that parliament will be sitting again this week, able to hold this Prime Minister firmly and fully to account.

"Unless, of course, he's done the decent thing before then and tendered his resignation."

On a fresh trip to the polls, she said: "I believe there should be a general election sooner rather than later.

"As soon as we have ensured a no-deal Brexit at the end of October has been averted, then a general election must happen.

"This situation cannot go on."

Addressing MSPs later in a parliamentary statement, the FM said Johnson had brought shame to his office.

She told Holyrood: "This behaviour on the part of a Prime Minister shames his office, it shames the UK government, it shames the Conservative Party and it demonstrates beyond doubt that Westminster politics is badly broken."

Speaking in New York, Johnson said the court's decision would not make delivering Brexit "much easier".

The Prime Minister said: "It is perfectly usual to have a Queen's Speech, that is what we want to do but more importantly, let's be in no doubt there are a lot of people who want to frustrate Brexit.

"There are a lot of people who want to stop this country coming out of the EU.

"We have a parliament that is unable to be prorogued, doesn't want to have an election, and I think it is time we took things forward.

"As the law currently stands the UK leaves the EU on October 31 come what may but the exciting thing for us now is to get a good deal.

"Obviously getting a deal is not made much easier against this background but we are going to get on and do it."

He added: "I don't think the justices remotely excluded the possibility of having a Queen's Speech but what we will certainly do is ensure Parliament has plenty of time to debate Brexit.

"Parliament has been debating Brexit for three years solidly - now is the chance for us to get a deal, come out of the EU on October 31 and that is what we are going to do.

"I strongly disagree with this decision of the Supreme Court.

"I have the utmost respect for our judiciary, I don't think this was the right decision, I think that the prorogation has been used for centuries without this kind of challenge."