A 60ft World War II pipe bomb has been found under a runway at a former Royal Navy air base.
Solent Coastguard uses HMS Daedalus for its helicopter fleet
The device is believed to be one of a number of explosives planted at HMS Daedalus, Lee-on-Solent, Hampshire, to be set off in case of invasion.
Hundreds of homes may be evacuated when the bomb is detonated in June.
Solent Coastguard and Hampshire police air unit, which both use the site, are still in full operation, but a glider club has been ordered to stop flying.
A spokesman for the police unit said it used a runway that was not affected.
Flying instructor Tony World, of the Portsmouth Naval Gliding Club, said the 18m bomb had been confirmed at the site during two months of recent survey work.
Battle of Britain
He said: "They are saying they are not sure that it is the only one.
"We have even been told we cannot walk on the grass."
The base was the headquarters of the Fleet Air Arm during the Battle of Britain.
Several explosive devices were found there about 15 years ago and were blown up.
Much of the base was recently handed over to the Maritime and Coastguard Agency (MCA), which had previously rented the site.
An MCA spokesman said: "As new landlords we are particularly keen to reach agreement with the gliding club based on new methods of operation we will be using the airfield for.
"We need to come to the agreement before allowing them to use the airfield."