There are “serious doubts” about coronavirus safety in government, the biggest civil servants’ union has warned, after Boris Johnson and six Conservative MPs went into self-isolation after being photographed standing near each other at a Downing Street event. The emergence of a new Covid cluster centred on No 10, with a total of 10 Tory MPs now isolating, has derailed Johnson’s hope of using this week to mark a fresh start following the infighting which saw his main adviser and media chief leave last week. It has also raised questions about Covid security inside Downing Street, with three MPs at the event having previously posted photographs of themselves standing about a metre from Johnson, with no one wearing a face mask. Johnson became seriously ill with Covid in April. Sixteen other people linked to Downing Street have either had a confirmed case of coronavirus, experienced symptoms or had to isolate since the start of the pandemic – including the health secretary, Matt Hancock, England’s chief medical officer, Prof Chris Whitty, and the former head of the civil service Sir Mark Sedwill. The flurry of feared cases began after Lee Anderson, the MP for Ashfield in Nottinghamshire, developed Covid-like symptoms a day after attending a meeting of 2019-intake Tory MPs on Thursday morning at Downing Street. A positive test on Sunday saw the prime minister informed, through an email sent by the test-and-trace service, that he must stay in self-isolation until 26 November because of his recent proximity to Anderson. Over the course of Monday, five more MPs announced they were self-isolating: Brendan Clarke-Smith, the MP for Bassetlaw in the east Midlands; Andy Carter, the MP for Warrington South; Lia Nici, the MP for Great Grimsby; Katherine Fletcher, who represents South Ribble in Lancashire; and Chris Clarkson, MP for Heywood and Middleton in Greater Manchester. Clarke-Smith and Carter had both posted photos with Johnson. Two No 10 aides at the meeting are also self-isolating. They were not named, but Johnson’s spokesman confirmed they were not Dominic Cummings or Lee Cain, the PM’s former chief adviser and communications head respectively, who both left Downing Street at the end of last week. Another four Tory MPs – Maria Miller, Jacob Young, Matt Vickers and Marco Longhi – have also said they are self-isolating. None was at the No 10 event, but Miller sat next to Anderson in the House of Commons last Wednesday. Johnson’s spokesman insisted Downing Street fully followed Covid-secure workplace guidelines, and that masks were not usually worn inside the building. He said the fact the MPs were photographed so close to the prime minister was mitigated by the fact they were stood side by side, not face to face. But a Public and Commercial Services Union spokesman said the incident prompted new worries, citing the case of the business minister, Alok Sharma, who faced questions about distancing in his office after a staff member tested positive for Covid. “From the controversy around Alok Sharma to parliament not having proper QR codes in place, our members have serious doubts as to the competency levels of ministers when it comes to following Covid safety measures,” he said. The FDA, which represents senior civil servants, said the case connected to No 10 was a “cause for concern”, saying it was vital for unions to play a role in ensuring government departments and parliament were safe. A spokeswoman said: “The risk assessments should include the measures the employer will take if there is a Covid outbreak in the workplace and in particular, employers need to be prepared to look again at the measures they have put in place if they have an outbreak – and if necessary make changes so that similar outbreaks don’t occur in the future.” Johnson has sought to play down the incident, saying he is fine and that his body “is bursting with antibodies”. His spokesman confirmed that the prime minister had had at least one antibody test for Covid, but did not say when this was. In a video posted to Twitter, Johnson said: “Hi folks, the good news is that NHS test and trace is working ever more efficiently, but the bad news is that they’ve pinged me and I’ve got to self-isolate because someone I was in contact with a few days ago has developed Covid. “It doesn’t matter that we were all doing social distancing, it doesn’t matter that I’m fit as a butcher’s dog, feel great – so many people do in my circumstances. And actually it doesn’t matter that I’ve had the disease and I’m bursting with antibodies.” Johnson’s spokesman said masks are not normally worn inside Downing Street because there are one-way systems in place instead to avoid close contact in confined areas. Asked why the meeting was held in person, he said: “Meetings for work are possible and are taking place. The PM has been having a mix of meetings.” Johnson, who must isolate for 10 more days – the 14-day self-isolation period begins with the last contact with the infected person – has been working in his Downing Street office rather than staying inside his flat in the building. His spokesman said Johnson was able to reach the office via a garden, with no contact with staff, and that this had been cleared by medical advisers. Downing Street said officials were in “advanced discussions” with parliamentary authorities about Johnson taking part in prime minister’s questions and other House of Commons proceedings remotely. Under the “hybrid” parliament arrangements, MPs can take part in some proceedings by video link.