The Prime Minister’s approval rating has also slumped from +6% to -6%, the Opinium study found.
The poll put the Tories on 42%, down two points from a fortnight ago, with Labour on 36%, up five points.
The figures are based on likely voters who chose a party.
The report came as it was reported Boris Johnson had married partner Carrie Symonds.
Labour leader Sir Keir Starmer’s net approval rating was -9%, the same as a fortnight ago.
Despite only a fifth of those surveyed saying they trusted Mr Cummings to tell the truth, many of the claims made during his seven-hour testimony to MPs were believed.
Some 66% said they believed the Government pursued a herd immunity strategy at the start of the pandemic, with 20% saying that was false.
The poll found 60% believed Mr Johnson thought coronavirus was a “scare story” and called it “the new swine flu”, with 24% saying that was false.
Some 56% believed Mr Johnson went on holiday in February 2020 without paying attention to the crisis that was approaching, with 26% saying that was false.
More than half – 52% – said they believed Mr Johnson was distracted by his personal life and was unable to concentrate on the decisions that needed to be taken, with 34% saying that was false.
Some 49% believed Health Secretary Matt Hancock lied about whether people would be tested before being discharged from hospital to care homes, with 28% saying that was false.
And 44% think Mr Hancock should resign as health secretary, compared to 30% who think he should stay.
Adam Drummond, Opinium’s head of political polling, said: “Whilst Dominic Cummings is seen as one of the least trustworthy men in Britain, the public do see some truth in the allegations he made against Boris Johnson’s government, puncturing the Conservatives’ post-Hartlepool bounce and reversing a recent spike in the prime minister’s approval rating.
“However, there are strong reasons to believe that this won’t last though as the underlying approval figures for the vaccine rollout, which has driven voting intention since the beginning of 2021, are unchanged and remain very strong.”
– Opinium Research carried out an online survey of 2,004 UK adults from May 27-28. Results have been weighted to representative criteria.