For almost 30 years they have been the silent guardians of the coast, stealing up on rogue vessels and reining in drug barons.

But now Searcher and Seeker are being put out to pasture, possibly to the island paradise of Zanzibar, as HM Customs and Excise updates its fleet of cutter craft.

The Home Office is steering its front-line ships into the 21st Century using £8 million capital raised through the Private Finance Initiative to pay for a new fleet which will be built in The Netherlands.

Doug McLellan, senior investigating officer at Customs House in Shoreham, said the two ships, which are completing their last tour of duty around the British coast, would be retired later this month.

He said: "They have a 20-year lifespan and they are now due to be replaced.

"Their primary objective is to locate all manner of vessels involved in the trafficking of class A drugs. These two are very well liked by their crews because they are capable sea boats and handle well in the worst weather conditions.

"We're giving them to the shipyard in part-exchange for the new craft and what happens to them then is in their hands."

The first brace of interceptors will leave Brighton in early June with the remaining five due to follow in the next few months.

Their ownership has been handed to Dutch shipbuilder Danen, which is building the replacements, as part of the deal.

Amani Abeid Karume, the president of Zanzibar, is reported to have visited Shoreham Port, where four of the Customs' seven ships are docked, to inspect the two interceptors with a view to taking them on to protect his own shores.

The two new craft will feature tracking technology to locate smugglers even further out to sea as they try to flood UK ports with drugs and bootleg goods. In 1999, Customs intercepted the import of more than £3.3 billion worth of drugs, 200 batches of firearms and explosives and almost 3,000 endangered live animals and birds.

A spokesman said: "With the increasing globalisation of the trade market, the growing international drugs threat and the quickening pace of technology, these are challenging times for the department and its 24,000 staff. The replacement of the cutter fleet is one of the ways in which we will keep pace with technology as it advances."

Dick Kellaway, chief investigation officer of HM Customs and Excise, at Custom House, London, said: "Seizures of drugs from yachts and other vessels in international waters are often the most effective way of guaranteeing the removal of large quantities of drugs destined for the UK.

"The evidence gathered on those concerned in the transport enables us to arrest and prosecute them without the presence of the drugs."