Former US President Bill Clinton has warned the Labour Party that its biggest challenge is having its achievements and ideas taken for granted.
Mr Clinton paid tribute to the Prime Minister and congratulated the Government on its "stunning success" in running the country.
And he applauded his likely successor as Prime Minister, Chancellor Gordon Brown, for his "brilliant economic leadership".
Mr Clinton, who last addressed a Labour conference four years ago, was given a standing ovation as Tony Blair introduced him as a "superlative politician, a tremendous leader of America - the one and only Bill Clinton".
But he warned delegates in Manchester that his experience showed that voters did not realise how swiftly things could change under a new administration.
To laughter, Mr Clinton apologised for making a "political point", adding: "I hope you'll forgive me for that - I don't get to do this very much."
He added: "I think your biggest problem right now is that people take your achievements and your ideas for the future for granted.
"I think the reason is that you have produced prosperity and social progress for so long it is easy for people to believe that it is just part of the landscape - that it's either going to happen anyway or if you get a set of new faces in the driver's seat surely they wouldn't change what's working."
Referring to his experience in the US, he added: "I say this to remind you that it can change quickly."
In an apparent swipe at the prospect of a resurgent Conservative Party he added: "Do not let anyone ever present to your citizens any future choice - as long as you are debating issues in the Parliament and when the time that comes you must debate issues in the election - as change versus more of the same. You are the change agents in this great nation. You have been and you will be."
