Mr Cameron said Mr Johnson was very serious about the job
Tory leader David Cameron has denied having to "tame" his party's mayoral candidate Boris Johnson.
"He's his own man, no-one tells him what to say or what to think," he told BBC Radio Five Live.
He said journalists expected "three gaffes a day" but Mr Johnson was very serious about the job.
Labour MPs have been told not to refer to Mr Johnson as "Boris". He and Labour mayor Ken Livingstone are among the few politicians known by their first name.
Henley MP Mr Johnson, who has one of the highest profiles at Westminster, is well known for his frequent TV show appearances and for controversial comments which include offending people in Liverpool and Portsmouth.
But his campaign has seen few jokes, with some commentators putting that down to campaign managers seeking to reduce any chance of gaffes.
Asked if he would be secretly glad if Mr Johnson did not win the mayoral contest - for fear of a run of gaffes in the run up to the next general election - Mr Cameron replied he would be "very sad" as London "needs a change" from Mr Livingstone - who has been mayor for eight years.
He said Mr Johnson had campaigned on the issues that mattered to Londoners - crime, dirty streets, transport and green spaces - and denied trying to rein him in during the campaign.
"I haven't been working to tame him at all. I've been out on the campaign trail with him.
"I think the press are a bit disappointed because they feel, because it's Boris, they are entitled to have three gaffes a day. The fact is that Boris is incredibly serious about doing this job.
"I think this is the moment when Boris realises he's got a great opportunity to do something for Londoners and I think he's fought an incredibly energetic and good campaign."
But he confirmed he did not support Mr Johnson's position on allowing an amnesty for illegal immigrants - adding that immigration was not a matter for the mayor.
The Conservative leader also said he would be telling all Conservative councils, in the run-up to the local elections, that Britain's number one priority was to keep the cost of living down and asked them to find "efficiencies" to keep council tax rises down.
Current mayor Mr Livingstone, who is also one of Britain's highest profile politicians, has warned against the mayoral contest becoming "Celebrity big mayor".
Minister Tessa Jowell has defended an instruction to Labour MPs not to refer to Mr Johnson as "Boris", saying she wanted to avoid the election being seen as a joke.
Meanwhile Lib Dem leader Nick Clegg has described his party's candidate - former police commander Brian Paddick - as the only "serious candidate" in the race.