Having experienced symptoms over the weekend Margaret Ferrier had ordered a Covid-19 test, but travelled to London anyway while waiting for her results.
Despite not knowing if she had coronavirus or not, the Rutherglen and Hamilton West MP then continued to breach the restrictions by attending parliament and giving a speech in the Commons.
At 7.15pm on Monday night Ms Ferrrier had stood up in the Commons and paid tribute to the hard work of NHS and care workers during the pandemic.
Getting a positive test later that evening, the SNP MP then got a train home from London anyway despite knowing she had coronavirus.
She now faces becoming the first person in the UK to be hit with £4,000 fine for "recklessly" leaving self-isolation.
Issuing a lengthy apology, she said: “I apologise unreservedly for breaching Covid-19 restrictions by travelling this week when I shouldn't have.
“There is no excuse for my actions.
"On Saturday afternoon, after experiencing mild symptoms, I requested a Covid-19 test which I took that day.
“Feeling much better, I then travelled to London by train on Monday to attend Parliament as planned. This was wrong, and I am very sorry for my mistake.
"On Monday evening I received a positive test result for Covid-19. I travelled home by train on Tuesday morning without seeking advice. This was also wrong and I am sorry. I have been self-isolating at home ever since.”
The SNP MP has now notified both the House of Commons authorities and the police over her actions.
Ms Ferrier, who is the party's manufacturing spokeswoman, added: "Despite feeling well, I should have self-isolated while waiting for my test result, and I deeply regret my actions.
“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."
The breach is a significant humiliation for Ms Ferrier, who will now face accusations of hypocrisy having previously demanded Dominic Cummings resign for breaking lockdown rules, and also called for a Cabinet Office investigation into his behaviour.
Speaking in May after it was revealed Mr Cummings broke lockdown rules, the Rutherglen MP had insisted his position was untenable.
She had said: “Dominic Cummings’ actions have undermined the sacrifices that we have all been making in lockdown to protect each other from coronavirus.
"His position is untenable and he must be removed from his post now.
"The public health advice is crystal clear.
"For the safety of others, anyone with coronavirus symptoms must self-isolate, in line with government guidance.
"They should not leave the house for any reason."
It is not yet known which London train station the MP used, or how she got to them from Westminster.
Trains to Glasgow go from both Kings Cross and Euston, either of which Ms Ferrier could have travelled to via a taxi or tube.
Labour MP Ian Murray accused her of putting people's health at risk.
He said: “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.
“To breach the rules twice is simply unforgivable, and has undermined all the sacrifices made by her constituents.
“Nicola Sturgeon must come out and condemn her MP’s actions and tell the Scottish people what disciplinary action she will be taking. There cannot be one rule for Margaret Ferrier, another for everybody else.”
The leader of Scottish Labour Richard Leonard added: "Margaret Ferrier’s reckless actions have shown utter contempt for transport workers, colleagues at Westminster, her constituents and the public."
Conservative MSP Ruth Davidson demanded she resign immediately and suggested this was worse than the breach by Mr Cummings.
She tweeted: “This Margaret Ferrier case is different - knowingly taking public transport after testing positive for COVID-19 is to put lives at risk. She has to go”.
Her actions were also criticised by the Train drivers union Aslef, who branded her actions “dangerous and disgraceful.”
Kevin Lindsay, ASLEF’s organiser in Scotland, said: “Margaret Ferrier travelled on a train to London after taking a test for the coronavirus, and then travelled back to Scotland when she knew she had tested positive.
“So she broke the rules twice, which is unforgivable, and put passengers and staff at risk, which is outrageous.
“For an MP to get on a train knowing she has covid-19 is both dangerous and disgraceful. “She should tell people which trains she travelled on, so those she put in danger can take appropriate measures – which is more than she did.”
The SNP is only believed to have found out about her symptoms and test today, fives days on from when she first felt ill.
Now leadership have now removed the whip from Ms Ferrier, with Nicola Sturgeon branding her actions “utterly indefensible”.
She tweeted: “It’s hard to express just how angry I feel on behalf of people across the country making hard sacrifices every day to help beat COVID.“The rules apply to everyone and they’re in place to keep people safe”.
Westminster leader Ian Blackford added: “I have spoken with Margaret Ferrier this evening who fully accepts that what she did was wrong.
“Margaret will be referring herself to the parliamentary standards commissioner as well as the police.
"I am tonight suspending the whip from Margaret.”
The House of Commons authorities told The Scotsman they were following the guidelines and had been informed over her positive test.
A spokesman said: “The House’s priority is to ensure that those on the estate are safe while business is facilitated.
"We have closely followed public health guidance on the action to take following a confirmed case of COVID on site.
"Parliament has a dedicated team to support the test and trace teams across the UK, acting as a central point of contact in the event of any suspected or confirmed cases, where an individual has been working on the Estate.”