Boris Johnson has been criticised for travelling seven miles from Downing Street to go cycling during lockdown.
The Evening Standard reported the prime minister had been spotted in the Olympic Park in East London on Sunday.
Government advice allows people to exercise outside, but says you should not travel outside your local area.
A No 10 spokesman would not confirm if Mr Johnson had been driven to the park or cycled there, but said the PM had complied with Covid-19 guidelines.
Labour's Andy Slaughter said: "Once again it is do as I say, not as I do, from the prime minister."
The Hammersmith MP added: "London has some of the highest infection rates in the country. Boris Johnson should be leading by example."
In response to the criticism, a Downing Street source told the BBC: "The PM has exercised within the Covid rules and any suggestion to the contrary is wrong."
A woman told the PA news agency she had seen the prime minister in the park: "He was leisurely cycling with another guy with a beanie hat and chatting, while around four security guys, possibly more, cycled behind them.
"Considering the current situation with Covid I was shocked to see him cycling around looking so care-free.
"Also, considering he's advising everyone to stay at home and not leave their area, shouldn't he stay in Westminster and not travel to other boroughs?"
Health Secretary Matt Hancock was asked at Monday's Downing Street press conference whether travelling seven miles for a cycle ride was within the rules.
Mr Hancock said: "It is OK, if you went for a long walk and ended up seven miles from home, that is OK, but you should stay local.
"It is OK to go for a long walk or a cycle ride or to exercise, but stay local."
'We need a solution'
The issue of travelling for exercise was highlighted at the weekend after two women said they were surrounded by police and fine £200 after driving five miles from home to take a walk.
Derbyshire Police have now dropped the fine and apologised to the women, but the incident led to a debate over the guidance.
Government advice for England says you can leave your home to exercise, but adds: "This should be limited to once per day, and you should not travel outside your local area."
The guidance adds: "Stay local means stay in the village, town, or part of the city where you live."
In Scotland, the advice is more precise, saying exercise can be taken if it "starts and finishes at the same place, which can be up to five miles from the boundary of your local authority area".
Former Liberal Democrat leader Tim Farron, who represents a constituency in the Lake District, has written to the PM calling for clearer guidance on exercise similar to that in Scotland.
He wrote: "On the one hand, our local police force here in Cumbria are reporting that people... have travelled hundreds of miles to take their exercise in the Lake District.
"And on the other hand, I have constituents writing to me, worried whether they will be punished for driving five minutes up the road to go for a walk in their local park."
Mr Farron added: "We need a solution that clearly deters people from making lengthy trips and potentially spreading the virus, but also that doesn't discourage people from keeping fit and healthy."