A key that could have saved Titanic has unlocked a 95-year-old mystery.

The tiny key, which carries a tag - Crows Nest Telephone Titanic - was for a cabin locker that stored the lookout's binoculars.

It belonged to David Blair, Titanic's original second officer, who sailed from Belfast to Southampton but was taken off the liner at the last minute after an officer reshuffle ordered by White Star Line management.

In his rush to leave the ship, Blair, from Newport, Isle of Wight, forgot to hand over the key and carried it off in his pocket.

This remarkable sequence of events almost certainly saved Blair and may have cost the lives of 1,500 passengers and crew - including 500 from Southampton.

As the ship departed Southampton, the lookouts reported that they had no binoculars in the crow's nest. It is now believed the binoculars were still locked inside the locker.

Days later, at 11.40pm on April 14, 1912, lookout Fred Fleet uttered the famous words: "Iceberg right ahead".

Fleet survived the sinking and at the official inquiry said that if he had binoculars he would have spotted the iceberg sooner.

A US senator asked Fleet: "Suppose you had had glasses such as you had on the Oceanic, or such as you had between Belfast and Southampton, could you have seen this black object a greater distance?"

Fleet replied: "We could have seen it a bit sooner."

Senator Smith: "How much sooner?"

Fleet: "Enough to get out of the way."

Blair only found the key after Titanic departed so kept it as a memento and passed it onto his daughter, Nancy.

The key is to be sold at auction next month and is expected to fetch up to £70,000.

"This little key could have played a part in the fate of the great liner," auctioneer Alan Aldridge said.

Also going under the hammer is a postcard Blair wrote to his sister-in-law expressing his disappointment at missing out on the maiden voyage.

Among the other 400 lots to go under the hammer at Henry Aldridge & Son auction house in Devizes, Wiltshire, is a rare Titanic launch ticket with an estimated value of £15,000.

The Titanic sale record was broken earlier this year when keys to the post room of the ill-fated Southampton liner sold for more than £100,000.

Meanwhile, negotiations are still under way for a mystery item - said to be the most valuable Titanic relic in existence - to be sold at auction in April next year, Mr Aldridge said.