MP Margaret Ferrier is believed to have attended Mass at a church in Glasgow after showing Covid symptoms.
The Daily Record reported that Ms Ferrier gave a reading as she joined up to 50 parishioners at St Mungo's RC church in Townhead on 27 September.
She later travelled to London before returning to Glasgow by train after testing positive.
Ms Ferrier has been widely condemned for risking the health of people in parliament and on public transport.
She has been suspended by the SNP and faces calls to quit as an MP. The Metropolitan Police are also investigating.
The Catholic Church in Scotland said it could not confirm whether Ms Ferrier, or anyone else, attended the Mass due to data protection laws.
A spokesman for the Archdiocese of Glasgow said: "For the good of the whole community, it is important that anyone who is required to self-isolate does so in accordance with the government's guidance, so anyone in that situation should not attend Mass.
"It is disappointing if this has not happened but we would like to reassure people that we fulfil all the government and church guidelines."
Police Scotland are not thought to be considering any action since self-isolation is guidance rather than a legal requirement in Scotland.
Under government Covid-19 restrictions, no place of worship should admit more than 50 people at any one time, regardless of its size or usual capacity.
Places at services are allocated via a booking system and everyone attending must wear a face covering.
Ms Ferrier, the MP for Rutherglen and Hamilton West, previously said she had experienced "mild symptoms" on Saturday 26 September and was tested for coronavirus.
However, she decided to travel by train to Westminster the following Monday before getting her result because she was "feeling much better".
She spoke for four minutes in the Commons chamber during a coronavirus debate - tweeting a video of her speech - but was told later that evening that she had tested positive for the virus.
Despite this, Ms Ferrier took a train back to Scotland on Tuesday 29 September, with SNP whips in the Commons being told about her positive test the next day.
It is understood she had initially told the party she was going home because a family member was unwell.
Ms Ferrier has apologised and said she "deeply regretted" her actions but has not yet given any indication of whether or not she intends to continue sitting as an independent MP.
She has referred herself to the Parliamentary Commissioner for Standards, as well as to the police.
Scotland's first minister, Nicola Sturgeon, has described Ms Ferrier's decision to travel to and from London when she should have been self-isolating as the "worst breach imaginable".
She said she had made it "crystal clear" to her that her "reckless, dangerous and completely indefensible" actions meant she should resign as an MP.