SNP leader Nicola Sturgeon has called on Margaret Ferrier to quit as an MP after she made a “monumental” error of judgment by taking a train from London to Scotland when she knew she had tested positive for coronavirus.

The Scottish First Minister said she had spoken to Ms Ferrier – who has had the whip withdrawn by the SNP – and made it “crystal clear” she should resign her seat, though the MP did not reveal what she intends to do.

Prime Minister Boris Johnson told BBC Scotland it is vital “everyone obeys the rules and the guidance” regarding coronavirus.

He said that in March and April, people in Scotland and across the UK “came together and got the virus down”, but he added: “Alas what happened since then is that everyone got a bit… complacent and a bit blase.

“I think everybody should stick to the rules and that’s what they are there for. We’ve seen they did make a huge difference.”

Speaking at her daily coronavirus briefing on Friday, the First Minister could not hide her displeasure with her former party colleague and said Ms Ferrier should “come to the right decision” and step down as an MP.

Ms Sturgeon said: “This was a monumental, actually almost incomprehensible, error of judgment on Margaret’s part, and I can’t make my feelings on that any clearer than I am doing.

“There surely can be nobody in this country who doesn’t know that if you have a positive test for Covid you should isolate yourself, and you certainly shouldn’t sit on a train for six hours taking a 450-mile journey.”

Ms Ferrier issued an apology on Thursday evening as she revealed she was tested for coronavirus on Saturday after developing symptoms, but then took a train to Parliament on Monday when she should have been self-isolating.

She spoke in a Commons debate on coronavirus on Monday, and was told at some point that evening she had tested positive.

Instead of self-isolating, she took a train back to Scotland on Tuesday.

DUP MP Jim Shannon, who was seated at the same dining table as her on Monday evening, with social distancing in place, is now self-isolating as precaution, having been given a negative test result on Thursday.

His party’s statement said the Speaker’s Office told him on Wednesday he was a close contact of a positive case and he immediately self-isolated.

SNP staff at Westminster say they were told on Wednesday Ms Ferrier had coronavirus, and initially thought she had taken the test after returning to Scotland – only learning about her breaches of self-isolation rules on Thursday.

Ms Sturgeon said on Friday that Ms Ferrier had been unable to give a “cogent explanation” for her actions.

The MP is now facing growing calls to quit, with Ms Sturgeon saying she has herself told her to resign.

The First Minister added: “I don’t have the power to force an MP to sit down, no party leader has that power.

“But I can make my views known and – difficult though it is – I have done so, and I hope she will come to the right decision.”

Ms Sturgeon said while coronavirus rules have changed during the course of the pandemic, “the one rule that has been clear and unchanging really throughout this is the need to self-isolate when you have symptoms and get tested, and if you test positive to absolutely make sure you complete that self-isolation”.

She added: “This is not a minor breach of the rules, it is not an inadvertent breach of the rules, it is a really flagrant and dangerous breach of the rules.”

Her tough stance was echoed by SNP Westminster leader Ian Blackford, who insisted Ms Ferrier’s position as an MP has been made untenable as a result of her “tremendous error of judgment”.

He told BBC Radio Scotland’s Good Morning Scotland programme: “I’m pretty angry at what has happened, it is not acceptable.

“It’s important that it is beyond reproach that everybody, absolutely everybody, must obey the rules and we have to think about the signal that gives the public and the sacrifices that people are having to make, and it’s in the light of that that Margaret will have to do the right thing.”

Commons Speaker Sir Lindsay Hoyle described her actions as “completely reckless behaviour”, and said he found out while in the Speaker’s Chair at around 4pm on Wednesday.

He told Sky News the reaction was immediate and “within 20 minutes we were in full swing on what we needed to do to ensure the safety and security of staff and members”.

A number of other SNP MPs, including David Linden whose Glasgow East constituency neighbours Ms Ferrier’s, have also called for her to quit.

Ms Ferrier, the Rutherglen and Hamilton West MP, said she took a test on Saturday afternoon after experiencing “mild symptoms”, meaning she should have self-isolated.

In a statement, she said she travelled home to Glasgow on Tuesday, where she has been self-isolating ever since.

She apologised for her actions and said there was “no excuse”.

Police Scotland said the MP informed them of her behaviour on Thursday and officers are “looking into the circumstances” along with the Metropolitan Police.

She could face a £4,000 fine for a first-time offence of coming into contact with others when she should have been self-isolating, under a law that came into force on the day of her positive test.

UK Communities Secretary Robert Jenrick told BBC Radio 4’s Today programme: “Margaret Ferrier has accepted that she has broken the law.

“That’s now a matter for the police, the authorities who are investigating the matter, and it is really for her to consider what her political future is, and for her party, the SNP.”

Scottish Conservative leader Douglas Ross branded Ms Ferrier’s decision to travel between London and Glasgow on a train with coronavirus symptoms as “reckless and dangerous”, adding that her position as an MP is now “completely untenable”.