Keir Starmer has described Boris Johnson as the “single biggest threat to the future of the United Kingdom” following his comments about Scottish devolution, during a history-making prime minister’s questions in which Johnson appeared via video link. The prime minister is self-isolating after a Conservative MP who attended a meeting at Downing Street tested positive for coronavirus, and answered questions from inside No 10 sitting at a desk. Starmer began by asking Johnson why, on a Zoom call with Tory MPs this week, he had described devolution as “a disaster north of the border” and “Tony Blair’s biggest mistake”. “Until now, whatever our disagreements, there has been a very broad consensus about devolution,” the Labour leader asked. “So why did the prime minister tell his MPs this week that Scottish devolution is, in his words, a disaster?” Johnson did not deny saying it, but avoided the question, arguing that “what has unquestionably been a disaster” had been the way the SNP had, he said, used devolution to push for a breakup of UK. Starmer responded: “The single biggest threat to the future of the United Kingdom is the prime minister, every time he opens his mouth on this. “When the prime minister said he wanted to take back control, nobody thought he meant from the Scottish people. But the prime minister’s quote is very clear – he said devolution has been ‘a disaster north of the border’. “This isn’t an isolated incident. Whether it’s the internal market bill, the way the prime minister has sidelined the devolved parliaments over the Covid response, the prime minister is seriously undermining the fabric of the United Kingdom.” Asked again about his comments, Johnson agreed that there were “great advantages in devolution”, pointing to his own period as the devolved mayor of London. But he appeared to back up his remarks about devolution in Scotland being problematic, saying that Tony Blair, who introduced it as prime minister, “has conceded that he did not foresee the rise of a separatist party in Scotland, he did not foresee the collapse of Scottish Labour”. Starmer used the rest of his questions to ask Johnson about the response to coronavirus, saying that the reason only about 11% of people fully abide by the 14-day self-isolation period is that many cannot afford to stay away from work. Johnson said the measures of statutory sick pay of £95 a week, or the one-off payment of £500 available to some people, was and “outstanding and exceptional” package of support, and accused Starmer of not backing the government over Covid. Starmer replied that he was “not going to take lectures on supporting”. He said: “The lockdown measures were passed the other week with Labour votes – 32 of [the PM’s] own MPs broke the three-line whip. And I hear that about 50 of them have joined a WhatsApp group to work out how they’re going to oppose it next time round.” The Labour leader also highlighted accusations of cronyism and a lack of transparency in private contracts connected to the supply of protective equipment and other Covid measures. His comments came after the National Audit Office said PPE suppliers with political connections had access to a “high-priority” channel for government contracts, where bids were 10 times more likely to be successful. Johnson replied: “All government contracts are of course going to be published in the due way, and are already being published. But I must say it is extraordinary that he attacks the government for procuring PPE in huge quantities.”