David Cameron should begin work on planning for the reconstruction of Syria, a charity said as a report estimated the civil war will have cost the country's economy up to £485 billion in lost growth if fighting ends this year.
World Vision estimated the current cost of the conflict is 275 billion dollars (£193 billion) in lost growth and urged the UK to lead international efforts to rebuild the country's shattered economy.
The charity compared the effort required to the Marshall Plan, the US initiative to support the rebuilding of western Europe after the Second World War.
The report examined the cost of the conflict for children, with schools and hospitals badly hit during the five-year war.
"Direct and indirect impacts on education services resulted in the equivalent of 24.5 million years of lost schooling by the end of 2015," the report said.
A quarter of schools have been damaged, destroyed or turned into makeshift shelters and around 5.7 million children need education assistance, according to the research.
Estimated life expectancy at birth has dropped by 15 years during the conflict and attacks on hospitals have left only 43% functional, while half the country's doctors have fled Syria.
The war has had a knock-on effect on the economies in neighbouring Lebanon and Jordan, the report produced by the charity and Frontier Economics found.
Tim Pilkington, chief executive of World Vision UK, said: "The war in Syria has shattered the lives of over eight million children. With their homes, schools and hospitals bombed and their friends and families killed, many have been forced into appalling living conditions and abject poverty.
"When we meet severely distressed children who've fled sniper fire and bombing, our first concern is to keep them fed, clothed and alive. Faced with their suffering it's hard to think in terms of cold economic costs.
"But financial loss translates into human loss - lost education, lost health, lost jobs and lost opportunities. The costs of the conflict are staggering. Unless we act now, this war won't just affect a generation of children, but their children's children.
"We cannot wait until the war is over to plan for their future. Every month the conflict costs Syria more than Britain's entire monthly budget for schools and education. Even if the fighting stopped tomorrow it would take at least 15 years to rebuild the country - an entire lost childhood.
"The UK has led the international community in humanitarian relief for millions of Syrian children and adults who have fled this vicious war. It would be a natural step for the UK Government to bring nations together on a historic restoration plan for Syria that will benefit us all."