Margaret Ferrier apologised for travelling from her Rutherglen and Hamilton West constituency south of Glasgow to attend the Commons on Monday.
She was experiencing symptoms of the virus and was awaiting the results of a weekend test at the time.
Astonishingly, the 60-year-old received a positive result on Monday night, and travelled 400 miles back to her Lanarkshire seat by train, without informing anyone of her condition.
In a statement tonight 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.
Margaret Ferrier said she was ‘very sorry’ for travelling from her Rutherglen seat to attend the Commons.
‘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.
‘I have used Test and Protect and I have notified the House of Commons authorities who have spoken with Public Health England. I have also notified the police of my actions.
‘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.’
A House of Commons spokeswoman tonight 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.
‘We are also implementing a number of cleaning measures to ensure those working on the Estate can maintain a clean and hygienic working environment and ensuring Parliament adheres to the Government’s ”Working safely during coronavirus” guidance.
‘Our risk assessment outlines the measures we have already put in place to reduce the risk of transmission in Parliament. The Speakers and political leadership of both Houses are keeping the situation under constant review.’