A drone aircraft was used by a Hampshire Royal Navy warship to spy on drug smugglers, leading to a series of raids in which heroin with a street value of £98 million was seized.

Six boats carrying narcotics were boarded off the east coast of Africa and their cargoes seized by an international task force which included British frigate HMS Richmond.

The Hampshired-based warship used its state-of-the-art unmanned aircraft - called ScanEagle - to fly undetected above the smugglers, monitor activity and pass back information to commanders of the international task force which is carrying out counter-terrorism and drug-smuggling operations in the region.

Ships from the Australian and New Zealand navies stopped the vessels as they headed for Tanzania and seized nearly one tonne of heroin.

A navy spokesman said that based on the UK current wholesale value of the illicit drug, the haul has a potential price tag of £26.5 million, but once cut several times it would be nearer £98 million as an estimated street value.

Despite not physically taking part in the boardings, HMS Richmond's Commanding Officer Commander Mark Anderson says his ship and its 200 sailors and Royal Marines had a vital role in the half-dozen seizures.

He said: "Richmond has played her part in disrupting the narcotics supply routes via Africa into Europe and the UK, working with France, Australia, New Zealand and Tanzanian authorities.

"It's a unique thing the Royal Navy can do, deploying thousands of miles from home, working side-by-side with regional allies and having a direct impact on the supply of narcotics into the UK."

HMS Richmond, based in Portstmouth, arrived in the region in April and will continue security patrols until the end of the year.