Peter Costello still harbours an ambition to become prime minister, according to the man who knows him better than most – his older brother, Tim.

In explosive comments, the World Vision chief last night said the former treasurer was “not totally extinguishing” a 30-year dream to lead the nation.

Tim Costello also believed there was a chance the Liberal Party would try to draft his brother into the leadership.

“There is no doubt that they treat him as really a Hercules. He has carried the weight of the last 11 or 12 years,” he said.

Mr Costello said his brother had been told, almost on a weekly basis, for the past 30 years that he would one day lead the nation.

“I suspect in a deeply personal sense you would still be not totally extinguishing that dream,” he said.

The strong endorsement came as the Liberal Party braced for the official launch of the Costello memoir, due to hit bookshops next week.

It reveals he was so certain that former prime minister John Howard was preparing to stand down that he prepared a wide-ranging speech during last September’s Asia Pacific Economic Co-operation summit in Sydney.

It mapped out a new direction for the then 11-year-old Coalition Government, including ratifying the Kyoto Protocol.

Peter Costello went on to accuse Mr Howard of failing to “accomplish generational change” in the Liberal Party.

His memoirs confirmed that he believed there was a decent chance Mr Howard would resign after APEC, giving a new Liberal team just months to lay out a fresh agenda before going to the polls.

Showing that he wanted to distance himself from Mr Howard, Peter Costello writes that he wanted to ratify the Kyoto protocol and outline “new ways to deal with the water crisis”.

He also planned new family and housing initiatives in a bid to win back voter support – only months before the November election.

“It was designed to set a new direction for a fresh government,” he writes.

He also takes a dig at Mr Howard for not following his hero – Robert Menzies.

“Unlike Menzies, Howard never managed a transition. He did not accomplish generational change.

“After the best economic record of any Australian government and after an age of prosperity from a golden era of continuous economic growth, the Coalition was defeated in the spring of 2007.

“We lost because we failed to renew. The electorate did it for us.”

Peter Costello yesterday formally ruled out challenging for the Liberal leadership but said there were still things he could do in politics.

“There’s some big issues coming up that I want to work on,” he said.

“The biggest issue in Australia at the moment is the economy is going down, people are feeling very insecure.”

There were also issues in his Melbourne seat of Higgins that he wanted to address, he said, adding that he was yet to decide about his future in politics.

“If I get to the stage where I feel that I can’t give it my full devotion, I’ll announce (my retirement) then,” he said.