Boris Johnson is not being tested for coronavirus, despite having attended an event with health minister Nadine Dorries who has the contagious disease, a Downing Street source has said.

Health chiefs are scrambling to find everyone who came into contact with Ms Dorries as parliament prepares to discuss possible restrictions.

Her contacts could potentially include ministers and officials in the Department of Health, MPs who shared the voting lobby with her last week, constituents at a weekend surgery and attendees at a reception hosted by Mr Johnson.

The No 10 source said that the PM was following the advice of Public Health England and the chief medical officer that a test was not needed because he was not in close contact with Ms Dorries at the reception and has shown no symptoms of the illness.

It is understood that Mr Johnson's fiancee Carrie Symonds, who is pregnant and also attended the event, has also not had a coronavirus test.

As nervousness grew at Westminster, a Labour MP revealed she had been told to self-isolate after meeting the health minister last week, telling colleagues: “It is so important that we all follow all public health advice.”

The authorities insisted there were no plans to suspend parliament “at present” and the Budget was set to go ahead as planned this afternoon.

However, officials will meet today, amid suggestions they wish to restrict visitors from entering parliament – while Downing Street still wishes to send out a “business as usual” message.

The coronavirus crisis deepened when the outbreak was revealed to have reached the heart of government, with Ms Dorries’ announcement that she had become infected last week.

Her Commons office was sealed off by the police, with dramatic notices reading 'COVID-19 DO NOT ENTER’.

The minister tweeted that she was “over the worst of it”, but the diagnosis could have severe consequences for the health department which is directing the nation’s response to the outbreak.

Ms Dorries is using a WhatsApp group to ask fellow MPs to let her know if they had “sat next to her in the tea room or library” in recent days, as she drew up a list of contacts to be traced.

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Matt Hancock, the health secretary, is believed to have told MPs not to self-isolate unless ordered to by a contact tracer – and made known he had not been contacted or tested and felt fine.

But Rachael Maskell, Labour’s shadow employment secretary, revealed: “NHS 111 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 coronavirus. Thankfully I am asymptomatic.”

The No 10 source said that Public Health England are urgently seeking to trace anyone who may have had contact with Ms Dorries, and will advise people whether a test or self-isolation is necessary. Their advice is that the virus can only be passed on to individuals within two metres of someone who has the infection.

Rory Stewart, the former Conservative Cabinet minister and London mayoral candidate, called for the Commons to be shut down – and the Budget “announced online” – but appeared to be a lone voice.

“MPs are at high risk through v frequent contacts and large gatherings. They are in danger of infecting each other in the chamber, and then going on to infect others. Time for Action. Now,” he tweeted.

At a pre-Budget Cabinet meeting, the chancellor insisted it would deliver on the nation’s needs despite coronavirus being “front and centre in our minds”.