The Ministry of Defence is storing more than 28 million explosives from cluster bombs, the BBC has learned.
They have been withdrawn from service and are to be destroyed. Some were only mothballed this year after the UK signed a treaty banning the weapons.
It is thought disposing of them will cost more than £30m. The UK government says it shows how concerned it is about the weapons.
But campaigners want countries like the US to ban them as well.
Cluster weapons can be fired by artillery or dropped from the air, and can take the form of rockets or shells. The UK used them in Kosovo and in Iraq in 2003.
Within the body of the weapon they contain small "bomblets" or sub-munitions which are spread over a large area.
As many as one in 10 do not explode when they land and remain dangerous to local people. Evidence from around the world suggests civilians make up 98% of the victims of cluster bombs.
In May this year the UK government agreed to sign up to a convention banning them. More than 100 other countries have said they would stop use, production and stockpiling of cluster munitions.
The final convention will be signed in Oslo in December this year.
Shells and rockets
The Ministry of Defence (MoD) has announced it will withdraw its last remaining cluster weapons; 56,000 artillery shells each containing 49 bomblets, and 4270 air-launched rockets with 9 bomblets in each.
That makes a total of more than 2,700,00 individual explosives.
Last year the MoD withdrew cluster weapons which were coming to the end of their life, containing about 28m bomblets.
However, it is thought to have destroyed fewer than 10% of those.
Officials say the disposal of the entire stock could cost between £30m and £40m. The treaty gives the UK eight years to complete the task.
The MoD says the disposal is "a sign of its commitment to addressing humanitarian concerns about the weapons".
A spokesman said the destruction of the cluster munitions "would not affect operational capability".
He added: "We are satisfied that we will be able to continue to play a full role in crisis management operations."
The move will also ensure "the necessary legal protection for our service men and women".
Campaigners have welcomed the move. Richard Moyes of Landmine Action said: "It sends a very clear signal that the UK government is committed on this issue."
However Mr Moyes points out the majority of UK cluster weapons went out of service before the convention on cluster bombs earlier this year in Dublin.
Despite the success of the treaty he says more needs to be done to lobby countries which did not sign the convention.
"We'd like to see the UK government putting pressure on the US to give up the use of these weapons," he said.
He believes there must also be "ongoing support" to clear areas where munitions were used but did not explode.