The prime minister's official spokesman told a Westminster briefing that Mr Johnson was in east London's Olympic Park to "exercise, which is in accordance with the guidance".
Government advice for England says people can leave home to exercise, but adds: "This should be limited to once per day, and you should not travel outside your local area".
But they did not confirm whether Mr Johnson had been driven to the Olympic Park from Downing Street or cycled there.
"He was leisurely cycling with another guy with a beanie hat and chatting while around four security guys, possibly more, cycled behind them", a witness told the PA news agency.
"Considering the current situation with Covid I was shocked to see him cycling around looking so care free", added the woman, who asked not to be named.
UK Prime Minister Boris Johnson has been slammed after it was reported that he enjoyed a bike ride in a park seven miles from his home.
Andy Slaughter, Labour MP for Hammersmith, west London, criticised the prime minister for having a "do-as-I-say, not-as-I-do" attitude.
Mr Johnson's press secretary Allegra Stratton, asked if Mr Johnson regretted his bike ride, said: "He will.be doing bike rides again - you all know how much he loves his bike".
Slaughter recalled that "London has some of the highest infection rates in the country", and stressed that "Boris Johnson should be leading by example".
Asked about Covid restrictions Mr Hancock was pressed on whether people should exercise seven miles from their home - thought to be a reference to the reports concerning Mr Johnson.
"If you're (...) genuinely taking exercise and not going out for other goal social purposes, then that seems perfectly reasonable".
She said: "Now, whether they all convert into fines that people have to pay is another matter, but we have issued 300 notices for a fine in 24 hours". And you should be two metres apart from the other person.
"We have set out clearly the rules". It's quite a lot and I think it will encourage more people to recognise we are in a health crisis.
Government guidance says daily outdoor exercise is allowed but people should not travel outside their local area.
"And, likewise, it is OK, if you went for a long walk and ended up seven miles away from home, that is OK".
Asked how she interpreted the term "local" regarding exercise, Dame Cressida added: "For me, a reasonable interpretation of that is that if you can... go for your exercise from your front door and come back to your front door".