Britain faces an unsustainable debt burden more than three times its current level unless future governments raise taxes to fund increasing costs from an ageing population and falling revenue from taxation on motor fuel, a watchdog warned on Thursday.

Britain's Office for Budget Responsibility, a government agency, said debt was on course to reach almost 320% of GDP in 50 years' time - up from 96% currently - if successive governments did not tighten fiscal policy.

"Factoring in a stylised estimate of the asymmetric costs associated with inevitable periodic shocks would push debt up to ... nearly 320% GDP in 50 years. These figures are based on a simple reading of post-war UK fiscal history," the OBR said. (Reporting by David Milliken, editing by Andy Bruce)