Dominic Cummings has launched an extraordinary new attempt to destroy the government’s credibility over Covid-19, claiming that ministers had backed a policy of “herd immunity” then lied about having done so. In an astonishing series of tweets on Saturday just days before he is due to appear before a Commons inquiry, the prime minister’s former adviser in effect accused the health secretary, Matt Hancock, of lying about the “herd immunity” plan and talking “bullshit” when he denied it to the media. Cummings also claimed that if “competent” people had been in charge of Covid strategy in its early stages, then it may have been possible to avoid the first lockdown, and certainly the second and third would not have been needed. In one tweet Cummings, who was accused in various media reports early in the pandemic of himself backing the idea of herd immunity – a policy allowing people to catch Covid in order to boost the number with antibodies – said: “Media generally abysmal on covid but even I’ve been surprised by 1 thing: how many hacks have parroted Hancock’s line that ‘herd immunity wasn’t the plan’ when ‘herd immunity by Sep’ was *literally the official plan in all docs/graphs/meetings* until it was ditched”. Turning his fire directly on Hancock, he added: “Yes the media is often incompetent but something deeper is at work: much of SW1 was happy to believe Hancock’s bullshit that ‘it’s not the plan’ *so they didn’t have to face the shocking truth*. Most political hacks believe in ‘the system’...” According to Cummings, who was ousted from Downing Street last November after a bitter power struggle and has since appeared intent on destroying his former boss, herd immunity was adopted as the official strategy in the early days of the pandemic. Another Cummings tweet said: “In week of 9/3, No10 was made aware by various people that the official plan wd lead to catastrophe. It was then replaced by Plan B. But how ‘herd immunity by Sep’ cd have been the plan until that week is a fundamental issue in the whole disaster.” Even before the latest Twitter broadside, Downing Street had been bracing itself for Cummings’ appearance before a joint Commons select committee inquiry into the handling of Covid-19 on Wednesday. He is understood to be ready to take his revenge campaign to a new level by producing incriminating texts, emails and WhatsApp messages detailing what he sees as serial incompetence in handling the response to Covid-19, while he was the prime minister’s closest adviser. Cummings was said to be particularly keen to focus on what he sees as Johnson’s reluctance to order a second lockdown last September. At that time Cummings, Sir Patrick Vallance, the chief scientific adviser, and Chris Whitty, the chief medical adviser were arguing for a short “circuit breaker” lockdown of a few weeks. But Johnson resisted. A series of heated arguments took place and the lockdown was put off until November, a delay Cummings believes cost many lives. His appearance before the joint inquiry of the health and social care and science and technology committees follows a spectacular bust-up with Johnson last month. This happened after the prime minister accused Cummings of leaking damaging information about him, including text messages and other information about the much-criticised refurbishment of his Downing Street flat. Cummings hit back in a blog post accusing Johnson of “unethical, foolish and possibly illegal” attempts to try to fund the home improvements with money from a Tory donor. A Downing Street spokesperson said: “Herd immunity has never been a policy aim or part of our coronavirus strategy. Our response has at all times been focused on saving lives and ensuring the NHS was not overwhelmed. We continue to be guided by the latest scientific advice.” They added that the prime minister had announced a public inquiry to learn any lessons that needed to be learned.