Audience choice is under threat, said Mr Richards
Ofcom has said the BBC must not be left as the only "public service" broadcaster, despite the worsening financial problems of commercial TV.
Ed Richards, the media regulator's chief executive, said ways must be found to help commercial broadcasters compete with the BBC.
He suggested one option could be to make part of the licence-fee available to them.
Mr Richards made the comments in a speech to the Royal Television Society.
The chief executive, who is leading a review of public service broadcasting, said responses showed almost no support for the BBC to be the only public service provider.
"Save, that is, for two private individuals who wrote in to support the 'BBC only' option. One began their letter by saying: "I don't have a television set, but…" he said.
Until now the main commercial broadcasters - ITV, Channel 4 and Five - have, in effect, received a public subsidy to provide news and current affairs, children's programmes and other public-service programming.
But that system is breaking down because of the growth of digital television and the internet.
Mr Richards said Channel Four faced a funding gap of between £60m and £100m by 2012 and ITV's financial position was worse than six months ago.
"The economic downturn is hitting all ad-funded business hard and online advertising looks to steal growth from more traditional media," he said.
Despite this, Channel 4 should continue to have a significant public service role in the digital age, building on its current contribution as a publicly-owned, not-for-profit institution, he added.
But Mr Richards said ITV1 and Five could one day give up any requirement to provide news and other public-service programming.
"Between now and 2014, ITV1 and Five should retain important public service broadcasting roles, focussed on UK origination and news and [for ITV1] the nations and regions," he said.
Mr Richards said the BBC should remain the cornerstone of public service content, and its core programme and services budget should be secure.
However audiences should have a choice of providers in most areas of public service content, which the market alone would not provide.
He said news, in its widest sense, was the most important area for competition.
"At the level of national and international news, the market may well deliver it," he said.
"But the picture is different in the nations and, to a degree, in the regions.
"Without a strong alternative voice to the BBC, there would be a real loss of plurality and a weakening of our interests as citizens."