Gordon Brown is to tell G8 leaders this week that the global financial situation may worsen.
The prime minister is expected to say that international complacency could hinder recovery and lead to a second downturn.
His warning comes as world leaders prepare to meet in L'Aquila, Italy on Wednesday for three days of talks.
"The world can not stand by and let events take their course," the prime minister will say.
"The need for co-ordinated international action to implement the decisions we have taken has never been more crucial.
"If we do not take the necessary action now to strengthen the world economy and put in place the conditions for sustainable world growth we will be confronted with avoidable unemployment for years to come."
Brown will deliver the warning during talks expected to focus on the world economy, climate change and international aid.
The UK is warning other nations that there are "many voices saying that the worst of the downturn is over, but there is no room for complacency".
Britain is to set out a five-point plan for boosting economic growth which includes increasing bank lending, restraint on oil prices, action against protectionism, increased investment and preventing a generation of young people becoming unemployed.
Business secretary Lord Mandelson added that Britain's work in Europe would have to be carefully monitored in the recession.
"We in Europe have to be mindful of the need not to become complacent and not to relax," he said.
"If we continue to get our reaction to the crisis right we can make Europe stronger."
