Keir Starmer has tested positive for Covid, the Labour party has confirmed, meaning he was unable to take part in prime minister’s questions and will not be able to respond to the budget. He has previously had to isolate four times during the pandemic. Ed Miliband took prime minister’s questions and Rachel Reeves, the shadow chancellor, is expected to respond to the budget. It is understood Starmer tested positive on a routine lateral flow test and is not feeling unwell. He was pictured earlier on Wednesday in Westminster with Reeves and the shadow chief secretary to the Treasury, Bridget Phillipson. Labour sources said both had tested negative and would not need to isolate as they were double vaccinated. Starmer’s deputy, Angela Rayner, who would have normally been his substitute at prime minister’s questions, is not in Westminster because of a bereavement. It is understood Starmer only took his lateral flow test 10 minutes before he was due to question Johnson and after a morning of preparation in Westminster. Miliband had to take over with minutes to spare. “But we didn’t have to ask him twice,” one source said. Miliband, the former Labour leader, was selected at short notice because of the forthcoming Cop26 climate summit, which falls under his brief as shadow business secretary. “We had always been planning to go on Cop at PMQs so Ed was the logical choice,” a source close to Starmer said. Reeves will respond to Rishi Sunak’s budget later on Wednesday, a task that traditionally falls to the leader of the opposition. Starmer has been repeatedly forced to isolate during the pandemic and his positive test will put him in isolation for a fifth time. Most recently he had to isolate in July after one of his children tested positive for coronavirus, he said. He also isolated in September and December 2020 and January this year. At prime minister’s questions Miliband joked it was “just like the old days” as he took on Boris Johnson for the first time over the dispatch box since regaining a frontbench role last year. “It’s one time only that I’m back,” he said, as Tory MPs jeered. Focusing on Cop26, Miliband said ministers had been shown to “face both ways” by striking a trade deal with Australia that allowed them to drop temperature commitments, telling other countries to rely less on coal while “flirting with a new coalmine” and urging them to turn away from fossil fuels while opening the new Cambo oilfield. He said the Glasgow summit was not a “glorified photo opportunity, it’s a fragile and complex negotiation”, and said the prime minister’s “boosterism won’t cut carbon emissions in half”. Johnson sent his sympathies to Starmer and said he hoped the Labour leader returned to the Commons soon. After being accused of downplaying expectations at the UN climate talks beginning soon in Glasgow, Johnson said progress was “unbelievably important for our planet”. However, he admitted: “Whether it is enough, I’m afraid it is too early to say.”