A handful of specially crafted glass valves each measuring one metre high are all that is stopping the historic home of Test Match Special, Yesterday in Parliament and the Daily Service going suddenly and permanently off air.

BBC Radio 4 long wave, which transmits on the 198 kilohertz frequency, relies on ageing transmitter equipment that uses a pair of the valves – no longer manufactured – to function.

The valves, at Droitwich in Worcestershire, are so rare that engineers say there are fewer than 10 in the world, and the BBC has been forced to buy up the entire global supply. Each lasts anywhere between one and 10 years, and when one of the last two blows the service will go quiet.

Last week, Mark Thompson, the director-general of the BBC, signalled the beginning of the end for the 198 long-wave service, which is still used by 90,000 homes in Britain to receive Radio 4 in areas where short-range FM does not penetrate.

Aware of the public sensitivity – the service fought off a closure threat in the early 1990s – Thompson promised that Radio 4 long wave would "find a new home" on both analogue FM and digital radio once suitable frequencies could be found.

Denis Nowlan, the network manager for Radio 4, said: "This is technology that is becoming obsolete. Digital radio now reaches 97% of the population, and there is plenty time to find new homes for long wave-only programmes."

Radio 4 was traditionally broadcast on long wave, using frequencies used by the BBC since the 1930s, but the station has long been aired on FM and digital radio and online. More recently, the long-wave service has been used to carry a handful of traditional programmes deemed unsuitable for FM, while the range of the long-wave signal also ensured that ships could pick up shipping forecasts.

The best-known programme broadcast on long wave is Test Match Special, which would otherwise dominate vast chunks of the Radio 4 schedule. Yesterday in Parliament airs when Westminster is sitting at 8.30am, cutting out the last 30 minutes of Today on FM, while Radio 4's Christian worship programme, the Daily Service, is carried every weekday at 9.45am.

So antique is the transmission equipment that the BBC does not believe it is possible to manufacture new valves because slightly faulty replacements could cause a catastrophic failure of the other parts of the transmitter. Whenever the valves fail a dangerous "arc of power" surges through the 700ft Droitwich transmission masts.

Building a new long-wave transmitter for Radio 4 would cost "many millions of pounds", according to BBC insiders. Part of the problem is that pumping the signal so that it can cover England, Wales and lowland Scotland requires 500 kilowatts of power, far more - according to the BBC - than other long wave transmitters, which makes the kit both unique and expensive. Meanwhile, modern mariners use other technology and services to get forecast information.

Highland Scotland is covered by two smaller transmitters and Northern Ireland does not easily receive Radio 4 long wave. However, the signal is strong enough to be audible in parts of the Netherlands, Ireland, France and Germany.

The BBC began national transmission with the National Programme, the predecessor of Radio 4, in 1926. Transmission moved to 200 kilohertz in 1934, when the BBC moved its transmission to Droitwich, and has remained at that frequency, allowing for a slight shift to 198Khz ever since.

Built under the leadership of Sir John Reith, his last act as director-general after being forced out in 1938 was to personally close down the National Programme at Droitwich before signing the visitors' book and leaving.

• This article was amended on 10-11 October 2011 to correct references to Droitwitch and Drotwitch, to Droitwich. A line saying that Droitwich's 500 kilowatts is far more than other long-wave transmitters has also been updated to attribute this statement to the BBC.