Britain is seriously underestimating the strength of the Taliban resistance to Western troops in Afghanistan, Defence Secretary Des Browne has admitted.

Mr Browne stressed the challenge the insurgents pose to British and other Nato forces in a speech to Royal United Services Institute.

He said the Government had been under no illusions that the operation would be tough but it had proved “even harder” than predicted. “The Taliban’s tenacity in the face of massive losses has been more than we expected.”

He also admitted success in Afghanistan seemed “some way off” given the current intense fighting.