Lifeboat coxswain Brian Thomson holds a speedline cable, like the one stolen
A lifeboat crew is warning that a piece of equipment stolen five years ago will now be like an "unexploded bomb".
The rocket-propelled speedline is used during rescues to shoot cable to people in the water.
RNLI chiefs said they fear for whoever has the stolen device, taken in a raid on the station's all-weather lifeboat.
Life-saving equipment at four Anglesey lifeboat stations, Holyhead, Beaumaris, Moelfre and Trearddur Bay, is being marked with forensic coding kits.
Lifeboat crew member and spokesman Ray Steadman said the kits had been bought as an attempt to deter thefts from the lifeboats and the lifeboat station.
In addition to the speedline, thieves who boarded Holyhead's all-weather Severn-class lifeboat one night five years ago also took a GPS system, binoculars, a night sight, a first aid kitand emergency food rations.
The raiders have not been traced but Mr Steadman said the concern was now if for the safety of whoever had the speedline.
He said: "The speedline is virtually a cannon. In the wrong hands, it would kill, if it hit somebody.
The chemical reveals a unique code under ultra-violet light
"When they used it, crew members had to anchor themselves to the boat's railings because of the recoil.
"It was a dangerous enough an item when it was new but now it will be beginning to break down and become unstable.
"Somebody is sitting with an unexploded bomb, if you like, in their shed, garage or under the stairs. There's quite a lot of concern about it. "
Life-saving equipment at four lifeboat stations is being marked with forensic coding kits to deter thieves.
Mr Steadman said Pc David Owen, whose brother and brother-in-law are Holyhead lifeboat crew members, have raised funds for the kits over the past 18 months.
The property-marking liquid shows up under UV light and gives a code unique to the location and the equipment on which it is dabbed.
The kits are been used at Holyhead, Moelfre, Beaumaris, and Trearddur Bay lifeboat stations on Anglesey.