Lt. Gen. Nick Carter, deputy chief of the International Security Assistance Force (ISAF), told The Independent daily that passing control of security to Afghan forces was going well.
But he warned any move to thin out British forces too soon would be unforgivable.
NATO combat operations against the Taliban in Afghanistan are scheduled to finish by the end of 2014.
And as part of a gradual withdrawal, the number of British troops in Afghanistan will be almost halved from 9,000 to 5,200 by the end of 2013.
Carter's warning came as forces for Afghan President Hamid Karzai called for support from ISAF during a battle against drug smugglers.
A major operation to airlift tonnes of British military equipment from Afghanistan, including helicopters and armoured vehicles, is already under way, BBC reported.
"It would be unforgivable if we allowed the gains of the last three years to be lost because we were not able to provide the Afghans with the support to take this through into 2014," Carter told the daily.
"Our judgement is we have to manage this in a way that retains confidence. Precipitating withdrawal that is not in line with the current plan will damage Afghan confidence."
He said the coming summer would be "a genuine test of the capability and confidence of the Afghan forces, a test of determination of the Afghan people to be with their government and a test of how much will remain in the insurgency".
At least 441 British personnel have died since operations began in Afghanistan in 2001.
--IANS (Posted on 01-04-2013)