Medals linked to one of the most enduring mysteries of a clandestine naval campaign waged off the East Anglian coast more than a century ago are up for sale.

The three First World War decorations belonged to Hugh Thompson, who vanished without trace in the wake of a clash between armed smacks and a German submarine made famous by the posthumous award of a Victoria Cross to Lowestoft fisherman Tom Crisp.

Thompson, a 22-year-old Gorleston-based Scot originally from Thurso, was among the six members of the crew of the Ethel & Millie last seen standing on the casing of the enemy U-boat after their vessel was captured and sunk.

His 1914-15 Star, British War and Victory Medals, which are being auctioned by Spink of London on Thursday, carry an estimated price tag of £240-£280, although such is the extraordinary story behind the young seaman’s final action that sum is expected to soar much higher.

What the catalogue entry for Lot 74 refers to as his probable “grizzly” end has long been the source of heated debate, amid claims that the crewmen were deliberately left to drown when the submarine ‘dived’, though proof of any war crime has never been substantiated.

No amount of conjecture, however, can mask the gallantry displayed by the crews of the two Lowestoft-based ‘Special Service’ smacks during their unequal fight with the UC-63 off the north Suffolk coast.

The Nelson, commanded by Tom Crisp, and the Ethel & Millie, skippered by Charles ‘Johnsey’ Manning, formed part of a fleet of disguised warships, so-called ‘Q-boats’, boasting hidden guns and small arms with which their volunteer crews set out to counter the German submarine menace.

Many of the men engaged in the dangerous ‘cat and mouse’ subterfuge were fishermen like Hugh Thompson. Married and living in Devey Cottages, High Street, Gorleston, he had enrolled in the Royal Naval Reserve in May 1915, serving aboard the trawlers Kingfisher and Halcyon II before joining the Ethel & Millie.

In keeping with their masquerade, the Nelson and Ethel & Millie were fishing off the Jim Howe Bank on August 15, 1917 when a submarine was sighted on the surface some three or four miles distant.

The action that followed with its heady mix of courage and controversy was destined to become enshrined in Lowestoft legend.

Hardly had Tom Crisp issued orders to ‘clear… for action’ than the enemy opened fire, sending up a plume of water roughly 100 yards off his port bow.

Determined to fight it out, Crisp maintained the pretence of a harmless fishing vessel while all the time trying to edge closer to bring the his smack’s much smaller guns within range of the submarine with Manning doing likewise.

But his brave efforts were all in vain. Moments later, a shell struck the Nelson below the waterline. Another tore through the mainsail, but even as the Nelson began to sink Crisp fought back.

Ordering his gun crew to open fire, they got off several shots, all of which fell short, before another direct hit struck Crisp, severing both legs and partially disembowelling him.

Despite his horrific injuries, he continued to give orders, telling his 18-year-old son, Thomas, who was serving as second hand, to make sure all confidential books and cyphers were thrown overboard and a message despatched by carrier pigeon giving their location.

Finally, with the sea lapping the deck, he instructed his crew to save themselves and to “throw me overboard”. Obeying the first order but not the last, the men of the Nelson pulled away in their ‘small boat’.

They were close enough to the Ethel & Millie to hear Manning call out for them to come aboard, but with the submarine now switching targets to bracket the remaining smack they decided to stay put.

As such, they were witnesses to the closing moments of the action. Looking on helplessly, they saw the U-boat score one direct hit before ceasing fire and slowing to pick up the smack’s crew.

What followed remains uncertain to this day. As the Nelson’s survivors rowed away in the opposite direction to be swallowed up by a fortuitous sea mist, they saw the Ethel & Millie men, Thompson apparently among them, lined up “on the submarine foredeck” seemingly taken prisoner. And that was the last time any of their fellow smacks men saw them alive.

While the country honoured the memory of Tom Crisp and the gallant Nelson with a posthumous VC and awards to two of his crew, the fate of Thompson and his shipmates continued to vex their families desperate for news.

Originally reported ‘missing’, they were officially ‘presumed dead’ the following March prompting renewed speculation about their likely fate. While some posited the theory that they may have been cast off in a ‘small boat’ and lost at sea, others imagined a more sinister end in which they were either murdered in cold blood or left to drown.

One supposition that can be definitively disregarded is that they were held captive but died when the submarine carrying them back to Belgium was mined and sunk before reaching port.

Post-war research showed that the UC-63 made it safely back to base, together with the Ethel & Millie’s gun which had been dismantled and a wealth of secret papers and intelligence, but no prisoners.

Indeed, the German submarine commander’s report made no mention of the smack’s crew ever being taken aboard his boat. Nor did it suggest summary justice having been meted out against men who he clearly felt had contravened the rules of naval warfare by fighting in “civilian dress” under the flag of the “English Merchant Navy”.

To the contrary, Oberleutnant zur See Karsten von Heydebreck was full of praise for his gallant adversaries in their hopeless struggle. “The two crews,” he wrote, “fought tenaciously and only abandoned ship after sinking following artillery [sic] hits or running out of ammunition.”

While it did little to resolve the riddle of the lost crew of the Ethel & Millie, it serves as a fitting epitaph to the desperate courage displayed by men like Hugh Thompson in one of the most celebrated naval actions of the First World War.

The sale of orders, decorations and medals takes place at Spink’s London auction rooms on Thursday starting at 10am.

Marcus Budgen from Spinks says: "The epic story of Q-Ships off the coast of Norfolk during the Great War is simply exceptional. The service and sacrifice of men like Hugh Thompson are a fine example of the great bravery shown against the odds and the fact he was closely linked to the famous Victoria Cross won by Thomas Crisp make this group very desirable to collectors. We expect a lot of interest from collectors when they come for auction on Thursday."