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.
Meanwhile, GPs are warning that routine appointments at surgeries may have to stop as the number of coronavirus cases rises.
The British Medical Association said routine monitoring of long-term health conditions might have to stop to enable GPs to “focus on the sickest patients”.
Ms Dorries, 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.
She 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. She was at a Downing Street reception hosted by Boris Johnson last Thursday.
Health Secretary Matt 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.”
Earlier, England’s deputy chief medical officer defended the decision to delay closing schools and introducing other stringent measures, saying experts were assessing new cases on an hourly basis to achieve a “balanced response”.
Dr Jenny Harries said she expected significant increases in the number of cases in the UK beginning in about 10 to 14 days time, at which point people with flu-like symptoms would be advised to self-isolate.
The vast majority of those diagnosed with coronavirus in Britain were “pretty well” but might “feel a bit rough for a few days”, she added.
Mr Hancock said that “wherever clinically and practically possible” people should access GP appointments “through phones and digital means”, rather than going to surgeries in person.
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.
The UK Foreign Office has warned Britons against all but essential travel to Italy, which is experiencing the worst outbreak outside China, after it introduced strict travel restrictions.
Italians are being told to stay home, seek permission for essential travel, and give justification if they want to leave the country.
The Foreign Office is advising anyone arriving in the UK from Italy since Monday evening to self-isolate for 14 days.
The government says it has facilities to accommodate Italian visitors to the UK who need to self-isolate.
British Airways has cancelled all of its flights to and from Italy until 4 April, and has asked staff to take voluntary unpaid leave.
Easyjet, RyanAir and Jet2 are also cancelling their flights on Italian routes, though EasyJet will operate “rescue flights” to bring British travellers home in the coming days.
‘Enter shops one at a time’
“It’s the weirdest holiday I think I’ve ever been on,” said Hannah Butcher, from Newbury, Berkshire, who is in Rome with her husband for their first holiday alone since having a child.
“We arrived on Sunday. The advice then was as long as you’re not going into Italy’s red zone, you’re OK.
“We’re currently sitting in a restaurant and everyone here is in staggered rows because they have to sit one metre apart. It’s quite weird seeing families spread across multiple tables.”
She added that people are “only allowed to enter shops one at a time”.
“All the attractions are closed; there are queues out the door of supermarkets and the butchers. There are police driving round making sure the rules are enforced and a noticeable armed police presence, presumably to keep order.”
She said they were due to fly home with Ryanair on Wednesday morning and had not been informed of any flight updates.
In other developments:
- London’s FTSE 100 share index failed to rally after a dramatic 7.7% drop on Monday, ending Tuesday in almost the same position it started in
- Great Ormond Street children’s hospital cancelled some non-essential cardiac operations for two weeks after a health worker in its cardiology department tested positive for the virus
- Prison campaigners at the Howard League for Penal Reform have raised concerns that “unhygienic” prisons could be “a centre for spreading the virus” as they called on the government to reveal its plan to protect inmates
- Royal Bank of Scotland, Lloyds and TSB banks said they would allow customers to defer mortgage payments if needed
- One of the UK’s biggest insurers, Aviva, has cut back on the cover available in new travel insurance policies because of coronavirus, with people not able to add cover for “travel disruption”
- The Church of England has urged parishes to stop contact during services – including sharing the chalice and shaking hands for the sign of the peace
- British nationals who were on board the virus-hit Grand Princess cruise ship are being flown back to the UK where they will go into self-isolation
- The NHS partnered with technology firms to help promote official health advice online
- Research shows it takes five days on average for people to show symptoms of the virus
- The NHS unveiled a range of measures – including NHS information to appear when people search “coronavirus” on Google – as part of an attempt to stop fake news being spread about the virus online
- A Holiday Inn hotel near Heathrow Airport has been reserved by the government for people arriving from abroad to self-isolate