British MP Nadine Dorries, a minister in the Health Department, tested positive for coronavirus, she said on Tuesday.
She said she was now "self-isolating at home".
Health officials are trying to trace where she contracted the virus and who she has been in contact with.
It comes after she attended a reception with Prime Minister Boris Johnson on Thursday before self-isolating on Friday.
But Downing Street said there are no plans to test Mr Johnson as he was showing "no symptoms" and had not been within two metres of the quarantined MP.
Ms Dorries had also given a speech at the House of Commons on Wednesday night and held a surgery for 50 of her constituents before falling ill.
The 62-year-old is also concerned she may have passed on the virus to her 84-year-old mother who has been staying with her after revealing she "began coughing" on Tuesday.
Ms Dorries, who helped craft the legislation to fight the virus, is the first British politician to have Covid-19 diagnosed..
Thanks for so many good wishes. 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. Keep safe and keep washing those hands, everyone.
— Nadine Dorries MP (@NadineDorries) March 10, 2020
No 10 Downing Street, the residence of the prime minister, did not comment on whether Mr Johnson had undergone testing, or whether he would be tested.
According to The Times, Ms Dorries fell unwell on Friday while signing the document that declared coronavirus a notifiable disease, meaning companies could obtain insurance cover.
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".
Another MP, opposition party Labour's Rachel Maskell, announced on Wednesday that she too is now self-isolating after she met with Mrs Dorries on Thursday.
She tweeted: "NHS111 have advised that I self-isolate as a result of a meeting I had with the Government's Mental Health Minister last Thursday who has subsequently tested positive for coronovirus. Thankfully I am asymptomatic. It is so important that we all follow all public health advice."
NHS111 have advised that I self-isolate as a result of a meeting I had with the Government's Mental Health Minister last Thursday who has subsequently tested positive for Coronovirus. Thankfully I am asymptomatic. It is so important that we all follow all public health advice.
— 💙Rachael Maskell MP (@RachaelMaskell) March 11, 2020
The news is likely to increase pressure to suspend parliament, despite the government's reluctance to do so in the middle of a health crisis.
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".
Six people have died in Britain from the virus, with more than 380 confirmed cases.
In France, culture minister Franck Riester has also tested positive for the virus and is self-isolating.
Comme vous le savez, j’ai été diagnostiqué positif au #coronavirus. Je travaille depuis mon domicile, en lien avec mon cabinet et les équipes du @MinistereCC. Je remercie très sincèrement toutes celles et ceux qui m’ont témoigné leur soutien et leur amitié.
— Franck Riester (@franckriester) March 10, 2020
On Tuesday it was announced that the heads of the Polish and Italian armed forces had tested positive along with members of Nato after a meeting at a German military camp.
More than 23 politicians in Iran have tested positive for the virus now.
One of its 11 vice-presidents, Masoumeh Ebtekar, had reportedly attended a meeting with President Hassan Rouhani and a number of cabinet ministers shortly before she was confirmed as carrying the virus.
It follows the news that its deputy health minister Iraj Harirchi, who led the government's Covid-19 task force, has also tested positive, along with the head of emergency medical services, Pirhossein Kolivand, and Esmail Najjar, who is responsible for the Interior Ministry's crisis management.
Iran's Industry, Mining and Trade Minister Reza Rahmani is presently in intensive care at Tehran's Imam Khomeini Hospital after contracting the virus.
Britain's parliament says it has no plans as yet to shut down to prevent the virus spreading.
"At present there are no plans to suspend parliament," a parliamentary spokeswoman said in a statement.
"We are closely following guidance from Public Health England in response to the situation and have been reassured that the measures we are taking are proportionate and appropriate."
