The defence secretary has been ordered by a judge to reconsider a decision not to protect a WWII wreck.
This summer the sisters visited the spot where their father died
The move comes as a major boost to families fighting to have the wreck, 10 miles off Hastings, East Sussex, protected as a war grave.
The daughters of naval gunner James Varndell - one of 21 men who died on the SS Storaa in 1943 - were granted the High Court order on Tuesday.
They had feared divers were at liberty to disturb their father's remains.
The torpedoed ship was taking steel to a weapons factory in Cardiff when she sank in November 1943, but the Ministry of Defence (MoD) ruled it was not a war grave.
Mr Varndell's daughters, Rosemary Fogg, 73, and Valerie Ledgard, 65, both of Worthing, said the MoD's decision that the ship was not in military service and so could not be designated a war grave was wrong.
James Varndell with his daughters and wife Amy
Lawyers for the sisters told Mr Justice Newman that the government was unlawfully failing to honour the memory of their father and the shipmates who died with him.
The Ministry of Defence argued that the vessel and the grave cannot be protected under the 1986 Protection of Military Remains Act because he died on board a merchant navy ship, not "on military service".
Other merchant navy veterans hope a victory by the sisters will lead to the protection of the graves of thousands of other sailors who died during World War II.
In the UK, 16 shipwrecks and all underwater military aircraft are designated war graves, which under the Military Remains Act 1986 imposes restrictions on their exploration and salvage.
John Short from Hastings Sub Aqua Association, who was sold the wreck for £150 in 1985, had argued the MoD had no right to turn it into a war grave because he was the legal owner.