David Cameron discussed "brotherly love" when he was questioned over the Labour leadership contest.
The Prime Minister admitted he grew up in the shadow of his older brother during a television interview, but was reluctant to say which of the Milibands he would prefer as opposition leader.
Earlier this week, Ed Miliband accused Mr Cameron of "hiding behind the deficit" and offering 'miserable pessimism'. In response the Prime Minister warned that Mr Miliband would "betray the country's national interest".
"To say what Ed Miliband has said, to say 'forget about the deficit' because it is somehow pessimistic to talk about it, that would be completely betraying the country," he said.
"If you are trying to do this job properly it is not about being popular, it is about trying to do what is right."