Ms Ferrier spoke in the coronavirus debate in the House of Commons on Monday and, by her account, tested positive for Covid-19 that evening.
“I travelled home by train on Tuesday morning without seeking advice. This was also wrong and I am sorry,” she said. “I have been self-isolating at home ever since,” she said.
The MP said she had “notified the police of my actions” and has informed the Test and Trace system and the Commons authorities.
“Despite feeling well, I should have self-isolated while waiting for my test result and I deeply regret my actions,” she added. “I take full responsibility and I would urge everyone not to make the same mistakes that I have, and do all they can to help limit the spread of Covid-19.”
Ms Ferrier’s actions were branded “unforgivable” by fellow MPs as she faced calls to resign.
Tory MP Alicia Kearns tweeted: “The number of people who have been put at risk, MPs and staff in Parliament and their families, and all those on public transport, and those who everyone has since engaged with. It’s utterly shameful, and to wait three more days to tell anyone is unforgivable.”
Labour’s shadow Scottish secretary, Ian Murray, said: “This is astonishing recklessness from an SNP MP, which has put people’s health at risk.
“Through her irresponsible actions, she very possibly has passed on the virus to a vulnerable person, who may now have Covid-19 and be in danger. She has put passengers, rail staff, fellow MPs, Commons staff and many others at unacceptable risk.”
The former Scottish Labour and Co-operative MP Paul Sweeney called for Ms Ferrier to resign, tweeting: “An MP travelling to London by train while awaiting a Covid test result is obviously a stupid thing to do.
“To then travel back to Glasgow by train after receiving a positive result is grossly negligent. Ms Ferrier should heed the advice she offered to Dominic Cummings, and resign.”
Ms Ferrier has previously criticised the Government’s handling of the coronavirus pandemic and urged the Cabinet Office to investigate Mr Cummings over his trip to Durham (see how that controversy unfolded in the video below).