Prime minister gives backing to general election debates but says they should be limited to candidates likely to win

David Cameron has voiced support for general election television debates, though he indicated that Nigel Farage should be excluded and they should start before the formal three-week campaign.

The prime minister said he thought the debates, which took place on three consecutive weeks in 2010, worked well though they took up too much time.

Cameron told the Andrew Marr Show on BBC1: "I want the debates to take place. I thought they were good in the last election. I thought they took up too much of the campaign so I think we could start them a bit earlier."

The PM, who recently turned down a challenge from Alex Salmond to debate the Scottish independence referendum, indicated that Farage should be excluded from the debates.

He said: "I think the debates predominantly should be about people who have a prospect of becoming prime minister." Asked whether he thought Farage would become prime minister, Cameron said: "I don't think so, no."

Some Tories believe that excluding Farage would kill off the debates. The Ukip leader would be likely mount a legal challenge to any plan to exclude him after next year's European parliamentary elections, which he is expected to win.