On the morning of 16 June 1955, Sidon was moored alongside the depot ship HMS Maidstone in Portland Harbour. Two 21-inch Mark 12 High test peroxide-powered torpedoes, code-named "Fancy", had been loaded aboard for testing. Fifty-six officers and crewmen were aboard.At 0825, an explosion in one of the Fancy torpedoes (but not the warhead) burst the number-three torpedo tube it was loaded into and ruptured the forward-most two watertight bulkheads. Fire, toxic gases, and smoke accompanied the blast. Twelve men in the forward compartments died instantly and seven others were seriously injured.
The submarine started to settle by the bows with a list to starboard, and her commanding officer, Lieutenant Commander Verry, ordered the submarine evacuated from the engine room and aft escape hatches. Thanks to a rescue party from Maidstone, everyone not immediately killed escaped, except Maidstones medical officer, Temporary Surgeon Lieutenant Charles Eric Rhodes. He had gone aboard with the rescue party, assisted several survivors, and suffocated because he was using a DSEA set that he hadnt been trained to use. At about 0850 Sidon sank to the bottom of the harbour. On 1 November 1955 Rhodes was posthumously awarded the Albert Medal for putting his life in danger to save others.
One week later the wreck was raised and towed into a causeway on Chesil Beach. The bodies of the 13 casualties were removed and buried with full honours in the Portland Royal Naval Cemetery on top of the cliffs overlooking the harbour.
A Court of Inquiry cleared anyone aboard Sidon for the loss of the boat. The direct cause of the accident was determined to have been malfunctioning of the "Fancy" torpedo. A torpedo being readied for the morning test shot had begun a "hot-run" - its engine had started while it was still inside the submarine and was over-speeding, creating very high pressures in its fuel system. The "Fancy" torpedo used high test peroxide (HTP) as an oxidizer. When an oxidizer line burst, HTP sprayed onto the copper fittings inside the torpedo, decomposing into oxygen and steam. The torpedos warhead did not detonate, but its hull burst violently, rupturing the torpedo tube and causing the flooding that destroyed the boat. The torpedo programme was terminated and the torpedoes taken out of use by 1959.
Sidon was refloated, then sunk to act as an ASDIC target on 14 June 1957.
On the 50th Anniversary of the Sidon accident, 16 June 2005, the Dorset Branch of the Submariners Association erected a Memorial Stone to those who died. This is situated adjacent to the War Memorial at Portland, opposite the Portland Heights Motel. A number of survivors and relatives of those who died in the accident attended the ceremony.