Boris Johnson could soon be forced into self-isolation after holding a 45-minute meeting with a Cabinet minister who became ill with suspected coronavirus shortly after.
The chancellor was also at the meeting and could also be forced to quarantine if Alok Sharma's Covid-19 test comes back positive.
The prime minister's spokesperson confirmed that even those who have had Covid-19 must isolate and when pressed on whether the PM would self-isolate for 14 days if told to, he said: “I would expect us to take medical advice and to follow it."
Business Secretary Sharma became ill just one day after the government scrapped virtual proceedings, forcing MPs to attend Parliament in person if they wish to participate.
But Number 10 says it will not review the return to physical-only proceedings, despite concerns that physical meetings could cause Mr Johnson and Rishi Sunak into isolation.
Many MPs had "serious" safety concerns about returning to Parliament, with many criticising the building's set-up as not fit for social distancing.
Despite Mr Sharma falling ill with suspected coronavirus, Brandon Lewis insisted to ITV News that Parliament has reopened "in a manner that follows the guidelines".
He claimed the new system - which sees MPs form a long, winding queue through Parliament in order to vote - ensures "proper good social distancing and protects people's health and safety".
"Although that does mean we have to queue a little bit to vote, I think it's right that we do that and the least we can do is queue up for a little bit to ensure that we are properly governing the country.
"That is actually, in the long run, hugely important for whole of the United Kingdom," he added.
But Labour's shadow foreign secretary said social distancing in the House of Commons is "impossible".
Lisa Nandy, who lost the Labour leadership election to Keir Starmer, said: “MPs are travelling home to every part of the country tonight. Reckless doesn’t even begin to describe it.”
She said the new queuing system, which has been set up to replace remote voting, has been a "complete shambles".
"It sends exactly the wrong message to the public about social distancing.
"The Palace of Westminster is just not suitable for social distancing, the online voting system was working relatively well, there was very little reason for the government to decide to bring MPs back to Parliament."
The new system sees hundreds of MPs forming a long, winding queue from the grass outside Westminster Hall to the despatch box in the Commons as they try to adhere to social distancing measures.
Northern Ireland Secretary Lewis gave an update on Mr Sharma, saying he may have just had "very severe hayfever".
He said the business secretary had not yet received the result of his test, but said "if he has coronavirus he will go into that track and trace system and anybody he's been in contact with will be contacted and they'll be given the recommendations to follow the guidelines."
A spokesperson for Mr Sharma said: “Secretary of State Alok Sharma began feeling unwell when in the chamber delivering the second reading of the Corporate Governance and Insolvency Bill.
“In line with guidance he has been tested for coronavirus and is returning home to self-isolate.”
During the debate, he was seen wiping his face with a handkerchief several times and his opposite number in Labour’s shadow cabinet, Ed Miliband, passed him a glass of water at one point.
A House of Commons spokesperson said “additional cleaning” had taken place following the suspected case of Covid-19.
The despatch box was being wiped down between exchanges, but the scheduled pause after the Bill’s reading went on for longer than expected and journalists were unusually asked to leave the press gallery.
Despatch Box is cleaned after Sharma speech
The size of the chamber has made it difficult for some MPs to keep their distance as they try to swap seats or move around.
Earlier in the day, Boris Johnson ended PMQs by heading towards the chamber exit but stopping for a chat with a Conservative colleague, thereby walking over hazard tape on the floor designed to encourage MPs to keep two metres apart.
Digital voting in the Commons was ended on Tuesday when MPs approved a government motion introduced by Commons Leader Jacob Rees-Mogg despite widespread objections.
Alok Sharma participates in the Commons voting queue
Senior Conservatives, opposition groups and the equalities watchdog raised concerns that the move would prevent many MPs, particularly the elderly and vulnerable ones who are shielding, from being able to vote.
There will be a vote on Thursday evening where MPs will decide whether to allow proxy voting for those who are "clinically extremely vulnerable or clinically vulnerable".
Mr Johnson on Wednesday dismissed complaints over the system, saying: “I do not think it’s unreasonable that we should ask parliamentarians to come back to this place and do their job for the people of this country.”
But Labour leader Sir Keir Starmer said the scenes were “shameful” and pushed the Prime Minister to end the “completely unnecessary and unacceptable” process.