Lib Dem MPs have discussed their leader's Commons performances

His Commons performances have been criticised in recent weeks and party frontbencher Simon Hughes said he needed more time to prove himself.

Sir Menzies told BBC 2's Daily Politics: "I'm aware that I would like to do better at PMQs."

But he said politics went beyond the weekly questions session.

And he suggested that Labour leader Tony Blair and Conservative leader David Cameron probably sometimes wished they had performed better.

Politics in this country is not prime minister's questions Sir Menzies Campbell Watch the Daily Politics

Sir Menzies told the programme: "Politics in this country is not prime minister's questions.

"It's important to all of us who live in the Westminster village, but it's not the issue on which people decide how they are going to cast their votes.

"That's not to say prime minister's questions isn't important. It is a shop window."

'Unhelpful time limits'

The 64-year-old former Olympic sprinter said leading his party was a tough job but he had never expected it to be easy.

"I think prime minister's questions poses particular challenges. I'm doing my best to meet those challenges," he said.

Mr Hughes has said the time to judge Sir Menzies is after six months as leader, at the party's autumn conference.

But Sir Menzies said: "I have set no time limits for my leadership and neither should anyone else."

Chris Huhne, who challenged Sir Menzies for the party leadership, said any talk of time limits was "frankly unhelpful and inappropriate".

"This is a bit of a summer silly season story," he said.