Health minister and Conservative MP Nadine Dorries has been diagnosed with coronavirus.
Ms Dorries, the first MP to test positive, said she had taken all the advised precautions after finding out and has been self-isolating at home.
This comes as a sixth person died from the virus in the UK, which has a total of 382 cases.
The latest person to die was a man in his early 80s who had underlying health conditions.
Meanwhile, Manchester City’s Premier League match against Arsenal on Wednesday has been postponed as “a precautionary measure” because of the outbreak.
A number of Arsenal players are in self-isolation after coming into contact with Olympiakos owner Evangelos Marinakis, who tested positive for the virus.
The government will unveil its first Budget later, amid growing fears about the impact the outbreak will have on the UK economy.
Chancellor Rishi Sunak has pledged the NHS will get “whatever resources it needs” during the crisis, while he is also expected to unveil measures to boost the self-employed and small businesses who are left out of pocket.
Meanwhile, NHS England said it was scaling up its capacity for testing people for the infection, with the number of cases set to rise.
Ms Dorries, the MP for Mid Bedfordshire, said in a statement that Public Health England had started tracing people she had had contact with, and the department and her parliamentary office were closely following its advice.
The 62-year-old, who began her career as a nurse, later tweeted that it had been “pretty rubbish but I hope I’m over the worst of it now”.
But she added she was worried about her 84-year-old mother who was staying with her and began to cough on Tuesday.
It is not known how many meetings Ms Dorries had attended at Westminster or in her constituency in recent days.
The Department of Health said she first showed symptoms on Thursday of last week – the same day she attended a Downing Street event hosted by the prime minister – and had been self-isolating since Friday.
No 10 did not comment on whether Boris Johnson had undergone testing, or whether he will now be tested.
All health ministers, including Health Secretary Matt Hancock, are to undergo testing for the virus, along with other officials who have come into contact with Ms Dorries.
Mr Hancock tweeted that she had “done the right thing” by self-isolating at home and wished “her well as she recovers”.
He added: “I understand why people are worried about this disease. We will do all we can to keep people safe, based on the best possible science.”