Health minister Nadine Dorries has tested positive for coronavirus and a member of her staff is ill.
Ms Dorries is self-isolating at home and has appealed to fellow MPs to get in touch if they have recently been in contact with her, Sky News understands.
Labour's Rachael Maskell revealed she had been advised by the NHS 111 helpline to self-isolate following a meeting with Ms Dorries last Thursday, even though she is displaying no symptoms of COVID-19.
She told Sky News she would continue her work as an MP by holding meetings over the phone, adding: "This will become a norm over the coming weeks and days as we are seeing more people being diagnosed with coronavirus."
Ms Maskell suggested Prime Minister Boris Johnson - who was at a Downing Street event with Ms Dorries last Thursday - should also now seek NHS advice.
"Everybody has got to take the advice of Public Health England and therefore I trust the prime minister has also contacted NHS 111," she added.
"There's nothing special about him, he's another person and like any of us, he could spread this virus.
"Therefore he must take that public health advice in the same way that I have taken the public health advice."
A Number 10 source said the prime minister has no coronavirus symptoms and so there is no need for him to be tested.
It is understood he was not in close contact with Ms Dorries at last Thursday's event.
Mr Johnson wished Ms Dorries a "speedy recovery" during a cabinet meeting on Wednesday morning, Downing Street said.
Ms Dorries, the minister in charge of patient safety, said in a statement that she "took all the advised precautions and have been self-isolating at home" since being informed she had tested positive.
"Public Health England (PHE) has started detailed contact tracing and the department and my parliamentary office are closely following their advice," she said.
"I would like to thank PHE and the wonderful NHS staff who have provided me with advice and support."
She added in a tweet: "It's been pretty rubbish but I hope I'm over the worst of it now. More worried about my 84yo mum who is staying with me and began with the cough today. She is being tested tomorrow."
Ms Dorries has also sent a message on the WhatsApp group for Tory MPs to ask them to get in touch if they were in contact with her.
The MP first showed symptoms on Thursday - the same day she attended the Downing Street event hosted by Mr Johnson to mark International Women's Day, according to a health department spokesman.
Ms Dorries has voted twice in the House of Commons in the past week, according to parliamentary records - likely meaning she has been in close contact with hundreds of MPs.
She also spoke in a debate on eating disorders in Westminster Hall, on the parliamentary estate, last Thursday.
However, there are currently no plans to suspend parliament.
The 62-year-old also met people at a constituency surgery in Bedfordshire on Saturday.
Health Secretary Matt Hancock said he was "really sorry" to hear the news but that "she has done the right thing by self-isolating at home".
He added: "We all wish her well as she recovers."
In other coronavirus developments:
A Department of Health and Social Care spokeswoman said Ms Dorries has been self-isolating at home since Friday.
Officials have begun the process of getting in touch with everyone she has come into contact with since displaying symptoms.
The DHSC spokeswoman initially said all health ministers including Mr Hancock were to undergo testing, but later retracted the statement.
It is understood Mr Hancock hasn't seen Ms Dorries since last Tuesday and has not been tested for coronavirus.
A government source said officials and MPs should continue to go to work in Whitehall and parliament unless told otherwise.
"CMO (Chief Medical Officer) advice is that no one needs to self-isolate or be tested unless asked to by the PHE contact tracers who are working round the clock," the source said.
After Ms Dorries' diagnosis, a parliament spokesman said there were no plans to suspend parliament to prevent the spread of the virus.
But they are "closely following" guidance from Public Health England, the spokesman added.
Hannah Bardell, an SNP MP, said that would be tough as "Westminster by its nature means we're all corralled in there together at close proximity".
She added: "It's possibly one of the most germ infested unhealthy places I've ever worked."
Sky News' political editor Beth Rigby said that "as a health minister in contact with other health ministers and officials the obvious question is whether there has been wider transmission in Department of Health team".
"I understand Matt Hancock is in good health and not displaying any symptoms," she added.
Politicians from across the political divide sent messages wishing Ms Dorries a swift recovery.
Labour shadow cabinet member Luke Pollard tweeted: "Politics should not get in the way of being decent human beings to one another."
Virus Outbreak: Global Emergency - Watch a special Sky News programme on coronavirus at 6pm weekdays