MP Margaret Ferrier will face no further police action for travelling from London to Scotland by train while knowing she had coronavirus.
Scotland Yard provided the update after the Rutherglen and Hamilton West MP referred herself to police over her actions.
The Metropolitan Police said Ferrier would be spared charges because new regulations on self-isolation came into force after she was tested.
However, the force has referred the matter to Police Scotland for consideration.
The MP took an 800-mile train journey from Westminster to Scotland after testing positive for Covid-19.
She had already travelled south by rail to attend parliament despite developing symptoms of the virus and having to book a test.
In a statement, the Met said: “Detectives tested the MP’s account including obtaining and reviewing CCTV and undertaking inquiries to establish the dates of the tests, results and travel arrangements.
“Officers considered possible offences including those under Reg 11(2) of the Health Protection Regulations 2020 which relates to self-isolation requirement.”
It continued: “However, on detailed examination of this new legislation, and following legal advice, it was concluded that this regulation is applicable only after the September 28, 2020.
“In this case the test occurred prior to September 29, 2020 and therefore the regulation does not apply.”
Ferrier has faced calls from her own colleagues to resign as an MP, including from First Minister Nicola Sturgeon, who has blasted her behaviour as “dangerous” and “incomprehensible”.
But in an interview with The Scottish Sun this weekend, she said she would not quit, and said she had been “hung out to dry” by the SNP after party officials scripted her initial apology statement
Ferrier suggested the SNP only withdrew the party whip after waiting to see what the public’s reaction would be.