ITV News Political Correspondent Paul Brand explains
Whilst the Prime Minister conceded the Tories are "not planning to get rid of all TV licence fees", he admitted the current system "bears reflection" and questioned "how long" the current system of funding the BBC can be justified.
Mr Johnson repeated his previous views, stating: "I certainly think is that the BBC should cough up and pay for the licences for the over-75s as they promised to do."
He later questioned whether the funding format for public service broadcasters "still makes sense in the long-term".
He said: "I think that the system of funding by what is effectively a general tax, isn't it, everybody has a TV, it bears reflection - let me put it that way.
"How long can you justify a system whereby everybody who has a TV has to pay to fund a particular set of TV and radio channels - that is the question."
The fee's existence is guaranteed until 2027, but what could change is the level at which it is set, due to be agreed in 2022.
In their manifesto, the Conservatives said they "recognise the value of free TV licences for over-75s and believe they should be funded by the BBC".
The Conservatives Party says in its manifesto it "recognises the value of free TV licences for over-75s and believe they should be funded by the BBC".
Both the BBC and the Government have been criticised over the announcement in June that free licences to all people aged over-75 were being scrapped.
Labour have promised to keep free TV licences for over-75s if they win power in the upcoming election.
Shadow culture secretary Tom Watson said: "This is a pathetic attempt by Boris Johnson to distract from his refusal to even look at the picture of a four-year-old boy forced to sleep on a hospital floor."
A BBC spokesperson said: "As we've said before, the licence fee ensures a universal BBC which serves everyone, is the most popular funding system among the public and is agreed as the method of funding the BBC for another eight years."
The row is the latest dig by the PM against the media after he declined to set a date for a sit-down with the BBC's Andrew Neil and failed to appear at an Channel 4 climate debate he was invited to.
Mr Johnson was instead replaced at the latter event with a melting block of ice.
Other countries, including several Nordic nations, have previously axed their own versions of the licence fee in favour of different funding models.