A boy who was less than a year old when his father died while diving off the northern coast of Scotland has won the right to sue a ship-chartering company for negligence.

In a unanimous judgment at the supreme court, five justices ruled that Scapa Flow Charters will have to defend the legal action following the death of Lex Warner, 50, near Cape Wrath in 2012.

The ruling reversed a previous decision in the Scottish courts that the claim was too late because the 1974 Athens convention, which governs the “carriage of passengers and their luggage by sea”, imposes a legal liability limit of two years after the date any passenger disembarked.

Warner had chartered MV Jean Elaine, a vessel operated by Scapa Flow Charters, for the week of 11 to 18 August 2012. When dressed in diving gear and preparing to dive to a wreck north-west of Cape Wrath on 14 August, he fell on to the deck of the vessel.

He was helped to his feet and went ahead with the dive, descending to a depth of 88 metres (289 feet). He got into trouble under water, however, and despite the assistance of other divers who brought him to the surface and on to the vessel, he could not be revived. He was pronounced dead onboard the ship.

The legal argument was over whether his son, Vincent, now six, should be allowed to sue. His mother, Debbie Warner, had not appealed against a lower court’s decision to bar her claim.

Scottish law provides for “the grounds of suspension and interruption of limitation periods” that would allow time limits for action to be extended.

Delivering judgment, Lord Hodge said: “In my view, the words in article 16(3) of the Athens convention … are sufficiently wide to cover domestic rules which postpone the start of a limitation period, as well as those which stop the clock after the limitation period has begun.

“In agreement with [Scotland’s] inner house, I conclude that Mrs Warner’s claim as Vincent’s guardian is not time-barred by the Athens convention.”

The legal action for damages against Scapa Flow Charters will now go ahead in the Scottish courts.

Debbie and Vincent Warner, who live in Birmingham, travelled to the supreme court in London to hear the judgment on Wednesday.

“This case needs to go to court. It’s very distressing. There are a lot of unanswered questions. I didn’t find out he was dead for 24 hours after what happened. He had life threatening injuries before he went into the water,” she said.

She also said she had been at risk of losing her home because of the problems of financing the legal challenge. She believed the Marine Accident Investigation Board (MAIB) report into the death had not exposed all of the issues raised by the accident. She said there had been other recent diving deaths off the north coast of Scotland.

The MAIB did find that there was a “significant risk to divers of tripping and falling when attempting to walk on the deck of a workboat in open sea while fully dressed and equipped for deep technical diving”. It also noted that wearing a large amount of heavy diving equipment can “significantly exacerbate the results of what might otherwise be considered a relatively minor fall.

“Although the skipper was aware of the demands of his working environment, there was no evidence of a formal assessment of the risks to a fully-dressed diver moving from his seated preparation area, to the point of entry into the water,” it said.