The row centres on how many homes will actually be able to receive the channel, due to start broadcasting at 6pm this evening.
Television sources predict that it will take months to get an accurate idea of who is actually watching the channel, as measured by the British Audience Research Bureau (BARB). "The problem is that the BARB sample audience may not be representative - there are just so many unknowns here," one said.
The confusion is likely to lead to a war of words between Channel 5 and ITV over whether initial viewing figures are accurate. If they are high, ITV advertising sales houses will question the accuracy of the BARB research. If they are very low - less than 4 per cent share of the audience - Channel 5 will do the same.
The core of the problem is that millions of viewers will find it hard to get a Channel 5 signal because they lie at the edge of transmission "footprints", because their aerials are not up to the task, or because their video recorders - which could interfere with the signal - have not been properly retuned.
So far the launch effort, including retuning in 10 million homes, has cost the main Channel 5 shareholders, Pearson and United News and Media, pounds 120m, more than the station's planned annual programming budget.
Two weeks ago the Independent Television Commission accepted assurances from Channel 5 that 90 per cent of homes that fell within the station's broadcasting range had been retuned. The station is claiming that its first broadcast will be available to 60 per cent of the British population.
However ITV sources dispute these figures, arguing that a variety of factors, including inadequate retuning by C5 engineers, problems with weak signals in some areas, and the fact that many households will think that the visit by the video retuner will mean that their television itself will not need tuning into the new station, will mean that C5 will be received by less than 40 per cent of British households.
Last month the Confederation of Aerial Industries estimated that 3.5 million homes would need a new aerial. More recently, C5 controller Dawn Airey stated publicly that the station "may go down the pan" because its audience will be too low.
The first audience figures, which will be available on Tuesday, are eagerly awaited by Channel 5 chief executive David Elstein, who is endeavouring to prove that the channel, with a much lower programming budget than any of its terrestrial rivals and a smaller potential audience, can still compete with ITV for advertising revenue.