Sharing the stage at the biennial convention of the Royal Television Society in Cambridge with the broadcasting minister, James Purnell, Mr Johnson made Channel 4's most explicit plea yet for urgent government assistance. He admitted that the broadcaster did not have an immediate financial crisis, but added: "The threats to Channel 4 will emerge in a very short time. To fail to protect it now would be a grave dereliction of duty."
Channel 4, which is funded by advertising revenues but publicly owned, has argued that as audiences inevitably fall as a result of the growth of digital television - which offers consumers more choice of stations to watch - its business model is no longer sustainable because it makes many non-commercial "public service" programmes. The broadcaster has said that it is looking at a funding gap of about £100m a year. To create extra sources of income, it has launched new channels - E4, FilmFour and later this year will come More4 - but this will not be enough to bridge the gap.
Mr Johnson said that before settling the new charter for the BBC "we need to ensure that Channel 4 can survive in the future". One option that he floated was the idea of giving Channel 4 some of the money that flows to the BBC from the licence fee. He also said that there were other ways that government could help through benefits in kind.
The former boss of Pizza Express told delegates: "It's not our ideal solution. I'm a capitalist and I'd rather we could survive our own."
He lashed out at the BBC and clashed with its director of television, Jana Bennett, who shared the stage with Mr Johnson at a conference session on Saturday morning. She could not see the case for sharing some of the BBC's £3bn-plus a year licence fee income. Mr Johnson was incensed that a television licence did not even mention the BBC.
"Does anyone here really believe they [the BBC] should get all of it [the licence fee]? They are all deeply embarrassed about it. It's a bit shameful to make your living from a poll tax."
Mr Purnell said the Government was "exploring" ways to help Channel 4 but, to Mr Johnson's frustration, the minister indicated that any assistance would be offered at around the time that the analogue television signal is switched off, that is around 2010 or 2012.
For the moment, Channel 4's finances are actually growing more healthily. Last year its turnover was £841.4m, up from £769.6m in 2003, while pre-tax profit was £65.7m, up from £45.3m.