Margaret Ferrier, the MP who broke Covid rules, clung to her position throughout Friday despite vociferous calls to quit from the SNP leader, Nicola Sturgeon, and other politicians. Downing Street was alone in not calling for the resignation of the Scottish MP, who spoke in the Commons while awaiting the result of a coronavirus test and then travelled by train from London to Scotland after testing positive. Having been suspended from the SNP on Thursday evening, she found herself in an increasingly isolated position by Friday. Sturgeon on Friday urged the member for Rutherglen and Hamilton West to step down, saying she had failed to offer her a “cogent explanation” for what she described as “a monumental, almost incomprehensible, error of judgment”. Labour also called for Ferrier to “do the right thing” and step down. Ferrier has reported her actions to police and the parliamentary standards commissioner. If she is convicted of a criminal offence in relation to the public health breaches, or sanctioned by the Commons standards committee, she could face a recall petition which, if signed by 10% of her constituents, would trigger a byelection. On Friday night the Metropolitan police said it had launched an investigation. In a statement, the force said: “An investigation is under way into reported breaches of the Health Protection Regulations 2020.” SNP sources said party colleagues were feeling “misled and betrayed” after Sturgeon confirmed that the MP initially told Westminster colleagues that she was returning home because a family member was ill. But, despite the flagrant breaches of coronavirus regulations in Scotland and England, Downing Street declined to call for Ferrier’s resignation, in contrast to the leader of the Scottish Conservatives, Douglas Ross, who has called for the SNP to expel her. “The prime minister has been clear that everybody needs to follow the rules in order to allow us to reduce the spread of the virus and protect lives,” Boris Johnson’s deputy spokesman said. “We have been clear people must self-isolate where they have coronavirus symptoms, if they test positive, or if they are contacted by test and trace.” Asked whether this meant Johnson thought Ferrier should resign, he said: “That’s a matter for her and her party.” The UK housing secretary, Robert Jenrick, likewise refused to call for Ferrier to resign, saying: “It is a matter for her to decide what she wants to do. It wouldn’t be right for me to comment on the police’s investigation.” Jenrick himself faced questions in April after the Guardian revealed he had travelled 40 miles to his parents’ home in Shropshire during lockdown after urging the public to stay at home. In an interview with the BBC, he was asked whether political figures flouting coronavirus guidance – such as Ferrier, Dominic Cummings and, more recently, the former Labour leader Jeremy Corbyn – made it harder to enforce public health messages because it gave the impression that the “Westminster club” was above the law. Johnson’s father, Stanley, has also apologised recently for breaching of coronavirus restrictions. At a daily briefing dominated by questions about Ferrier, Sturgeon hit back at “self-righteous criticism of the SNP” by people who “completely lost their tongue when a certain special adviser in London broke the rules”. Asked whether Ferrier’s actions had bolstered the sense that there is one rule for politicians and another for everybody else, Sturgeon said this was not the case. The Scottish first minister said: “I insist on the consequences when a politician breaks the rules being as severe as when it is a member of the public … “The most important relationship I have right now is with the Scottish public and I can’t ask you to make all of these sacrifices if I am going to stand here trying to explain away what Margaret Ferrier did.” Sturgeon also gave a detailed timeline of how news of Ferrier’s rule breaches emerged. “My understanding is that the SNP at Westminster knew on Wednesday that she had tested positive, but believed that she had taken a test once she had arrived back in Scotland,” the SNP leader said. “I understand that she had informed colleagues on Monday evening that she was travelling back to Scotland because a family member might have been unwell. “It was not until yesterday – and this information came through the Commons test-and-trace mechanism – that colleagues realised she had taken the test before travelling to London and then travelled back having been told that she was positive.” Sturgeon said she first heard about the situation on Thursday afternoon and was told that the Commons first wanted to put out a statement confirming that an MP had Covid. “I made clear if that was to happen, Margaret should immediately issue a statement confirming it was her and what the circumstances were.” Sturgeon added: “I think the SNP has acted quickly, appropriately and actually we have not tried to protect a colleague here, we have tried to do the right thing.”