Media sources warned revenues are set to decline by 8% in October and 7% in November, following a 5% fall in September. December, a less important month, looks to be down 1% at this early stage.
There are suggestions ITV will be worst hit due to comparisons with the Rugby World Cup in October last year.
Sources say the number of advertisers on TV has dropped and some consumer-goods groups are running fewer ad campaigns compared with last year as they focus on their core brands.
The run-up to Christmas is the most important quarter of the year and TV is seen as a bellwether for the wider market. A TV source said: "November looks to be the most significant because it is the first ‘clean’ month since Brexit with no unusual comparative with last year."
ITV first warned of a Brexit effect in the spring ahead of the referendum in June. Last week, Sky said its UK ad revenues fell 3% in the three months since the vote but added that was "slightly better than the overall market".
ITV declined to comment.