LONDON--British opposition leader David Cameron on Saturday kicked off his campaign to become prime minister this year, saying the country could not settle for five more years under Gordon Brown.

In a speech focused on broad ideas rather than policy detail, Conservative Party chief Cameron said the governing center-left Labour Party seemed set on fighting a class war at the next general election, due by June.

Cameron said the Conservative election campaign was up and running as of Saturday, and the first chapter of the party's manifesto would be published next week.

"The next general election is no more than 153 days away and as far as I'm concerned, it cannot come a moment too soon," the 43-year-old said in a speech in Oxfordshire, central England.

"Let us make this the year for change; the year when the positive defeats the negative; because one thing is absolutely clear: we cannot go on like this.

"We need change to help this country back on its feet again. And this change needs to be based on two clear principles: supporting aspiration and rewarding responsibility."

He attacked Labour's "irresponsible economic policies" which he said threatened Britain's recovery from recession.

"We can't go on with an old-fashioned, left-wing class war on aspiration from a government that has actually seen the rich get richer and the poor get poorer," he said.

"We cannot go in these difficult times with a weak prime minister and a divided government. We cannot go on for another five years with Gordon Brown."

Cameron also unveiled plans for a cross-party "war cabinet" to oversee the military campaign in Afghanistan, with opposition leaders invited into top-level meetings "on a regular basis" to consider the war strategy.

"When a nation is at war it needs to come together and to pull together," Cameron said.

The centre-right Conservatives last won a general election in 1992 under prime minister John Major. Labour took power under Tony Blair in 1997, with Brown taking over in 2007.

"If we win this year's election Britain will be under new economic management," Cameron said.

"We will send out the loudest signal that this country is back open for business and ready for investment.

He said a Conservative government would "redistribute power from the political elite to the man and woman in the street", with the "most radical decentralization of power this country has seen for generations".