LONDON (Reuters) - An Afghan policeman has shot dead five British soldiers at a checkpoint in southern Afghanistan, the defense ministry in London said on Wednesday.

The men, three from the Grenadier Guards and two from the Royal Military Police, were killed at a police checkpoint at a military compound in Helmand province on Tuesday.

"An Afghan national policeman from the checkpoint started firing without warning before anyone could really respond," a defense ministry spokesman said. "Every effort is being put into hunting him down."

A British military spokesman in Kabul said the gunman may have been working with another man and the British Royal Military Police and the Afghan National Directorate of Security were investigating.

Prime Minister Gordon Brown said the whole country would mourn the loss of the soldiers.

"The death of five brave soldiers in a single incident is a terrible loss. My thoughts, condolences and sympathies go to their families, loved ones and colleagues," Brown said.

"They fought to make Afghanistan more secure, but above all to make Britain safer from the terrorism and extremism which continues to threaten us from the border areas of Afghanistan and Pakistan."

Afghan election officials canceled a presidential run-off election on Monday after Afghan President Hamid Karzai's rival, former Foreign Minister Abdullah Abdullah, withdrew citing serious concerns about the election.

"It is my highest priority to ensure our heroic troops have the best possible support and equipment - and the right strategy, backed by our international partners, and by a new Afghan government ready to play its part in confronting the challenges Afghanistan faces," Brown added in his statement.

(Reporting by Peter Griffiths in London and Jonathon Burch in Kabul)