Health minister and Conservative MP Nadine Dorries says she 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 had been self-isolating at home.
It 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.
Confirmation of any positive test results will also be accelerated with most people getting a result back within 24 hours.
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 – 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.”
The number of total cases for the UK include 324 cases in England, 27 in Scotland, 16 in Northern Ireland and 15 in Wales.
There are 91 in London, with the next highest infected area being south-east, with 51 cases. Cases by local council area in England can be viewed here.
The latest person to die, on Monday evening, was a man with underlying health conditions who was being treated at Watford General Hospital.
He caught the virus in the UK and officials are trying to trace who he had been in contact with.