Dorothy Byrne delivered a keynote speech at the Edinburgh TV Festival on Wednesday, calling Prime Minister Boris Johnson a "coward" for not granting news interviews.
She also said he was "a known liar".
Byrne told the BBC: "If someone has deliberately spoken an untruth and we have evidence, we have to consider saying that what is said is a lie."
In her MacTaggart Lecture, she said: "What we all need to decide: what do we do when a known liar becomes our Prime Minister? I've talked to journalists from several television organisations about this issue. They said they would be loath to use that word 'liar'.
"Remember when Andrew Marr told [former defence secretary] Penny Mordaunt her claim that the UK couldn't stop Turkey from joining the EU was 'strange'?
"It was strange, but it was also untrue - a lie. Is it time for us to start using the L-word? I believe that we need to start calling politicians out as liars when they lie. If we continue to be so polite, how will our viewers know that politicians are lying?"
She added that it "isn't necessarily obvious" to the audience when politicians are untruthful in interviews.