After a tightening of the polls in recent weeks, the Conservative leader acknowledged that it would be a "close fight" to the finish.

But he insisted his party had a once-in-a-generation opportunity to get into power at the expected May 6 election.

"This election is going to be a tough fight, it's going to be a close fight," he said.

"I can tell you they will throw everything at us that you could ever imagine."

Surrounded by Conservative supporters at the Milton Keynes Academy, Mr Cameron went on: "So, my friends, 40 days and 40 nights.

"I don't want you to think of them in terms of weekends or bank holidays or anything like that. It's 40 days, it's 40 nights, and they're all the same.

"That is the time that we've got left to win this great argument - do you want to stick with what you know or do you want change with the Conservatives.

"And we've got to convince people that frankly right now we are stuck with what we know. The economy's stuck, society's stuck, the whole country is stuck with Gordon Brown.

"And we need that change, that energy, that dynamism, to get our economy moving, to get our country moving, to get our society moving."