She acknowledged that “there is no excuse for my actions” after revealing her journey while positive with the virus.
Ms Ferrier said she took a test on Saturday afternoon after experiencing “mild symptoms” - meaning she should have self-isolated - before travelling by train to London on Monday.
Amid calls for Ferrier to resign, it has been revealed she previously criticised Boris Johnson’s senior adviser for his conduct during the pandemic.
Speaking during Cabinet Office questions on June 11, Ms Ferrier told the Commons: “Public health experts have voiced concerns that Dominic Cummings did undermine public trust in lockdown rules, going against the principle of integrity that is in the code of conduct.
“Will the Cabinet Office conduct an investigation into potential breaches of the code of conduct by Mr Cummings, or have ministers yet again decided that they have had enough of experts?”
The subject of the questions at that point in the session was the code of conduct of special advisers.
Mr Cummings became engulfed in controversy in May after it was revealed he travelled 260 miles from London to his parents’ farm in Durham because he suspected he and his wife may have had coronavirus.
In an attempt to draw a line under the row, he made a public statement a few days later and has since managed to keep his job.