The members of the King's Troop, Royal Horse Artillery, were caught after a routine test of their unit, whose main ceremonial role is to perform gun salutes to mark the Queen's official birthday and other royal occasions.

An MoD spokesman would not discuss the drug involved or the disciplinary action taken against the five but it is believed they were all sacked from the Army after a Class A drug was discovered in their samples.

He said: "We can confirm that five members of King's Troop, Royal Horse Artillery, London, have provided positive samples following their unit's recent testing as part of the Army's drug testing programme. Internal administrative action has been taken against all five soldiers."

The MoD spokesman said: "Drug misuse in the Armed Forces will not be tolerated.

"Positive rates in the Army over the last three years average around 0.8 per cent, compared with over seven per cent in civilian workplace drug testing programmes in the UK."

Reports said two of the five dismissed were women and two were Iraq veterans.

For the past three weeks the King's Troop has been guarding Buckingham Palace. This duty ended yesterday.