Penalties written up for more than 20 businesses in the gym sector have been waived in recent months.
Lucinda Nicholls, who successfully represented the 23 firms and is still fighting on behalf of seven more, said evidence presented against her clients was seriously deficient.
The validity of the penalties – issued to at least 100,000 individuals and about 900 businesses – has also been called into question after Mr Johnson claimed in his Partygate defence that he was confused by his own government’s rules, she added.
Ms Nicholls, a lawyer from the London-based law firm Nicholls & Nicholls, said: “Boris Johnson said he found the regulations confusing. Considering they changed about 12 times, it is understandable someone might be confused.”
“In the context of Partygate, the government should be overhauling all of the Covid fines issued to individuals and businesses,” she added.
“If the government that brought in the legislation had difficulties with regard to the exemptions and the defences, then that demonstrates that there is clearly going to be confusion among the local authorities and the police in relation to enforcement.”
Police in England and Wales issued 877 notices under business regulations while more than 100,000 were imposed on people over various other allegations, data released by the National Police Chiefs Council in March showed. Local councils have also issued similar fines.
Gainz Fitness & Strength, an independent gym in Bedford, was one of the firms Ms Nicholls has represented.
The gym’s owner, Alex Lowndes, risked being fined £10,000 when he decided to stay open in November 2020 after rules ordering gyms to be closed had come into force.
Mr Lowndes told the BBC that the council case was “flimsy” and said that it was also “ludicrous” to continue to prosecute gym-goers after the scandal emerged that Downing Street hosted a number of lockdown-busting parties.
He also told the broadcaster: “But they kept going and they kept going, they brought in an external barrister, they kept spending money, and it just got out of control.”
Mr Lowndes was due to stand trial last March. But the council failed to gather sufficient evidence and magistrates rejected its request for the trial to be adjourned.
Ms Nicholls said there was no prospect of appeal where the penalty had been paid, since this represented an admission of guilt, and said ministers should act to ensure all were overturned.
Bedford borough council, which gave Lowndes a fixed-penalty notice, said regulations were enforced in line with its duty at the time.