Auctioneer Andrew Aldridge believes the cablegram, relayed to Harland and Wolff, could fetch several thousands of pounds when it goes under the hammer this September.

The document brought to Belfast City Hall describes how Belfast-born designer Mr Andrews fought to save lives and was last seen throwing deckchairs overboard so that people in the icy water would have something to clutch on to.

The poignant passages also commend the heroism of Belfast shipyard workers in the guarantee group on board the doomed liner who remained below " doing their duty" as Titanic sank beneath the waves.

Mr Aldridge also described the "Antiques Roadshow moment" as another visitor showed him a launch ticket for the Titanic. This is expected to fetch between £6,000 and £10,000 when it is auctioned alongside the cablegram by Henry Aldridge & Son at Devizes on September 22.

The telegram was brought in by a woman who has been collecting Titanic documents with her husband for many years, Mr Aldridge said. Harland and Wolff received it on April 23, 1912, eight days after the tragedy.

The cablegram reads: "After accident Andrews ascertained damage advised passengers put heavy clothing prepare leave vessel. Many sceptical about seriousness damage but impressed by Andrews' knowledge personally followed his advice saved their lives.

"He assisted many women children to lifeboats. When last seen officers say was throwing overboard deckchairs other objects to people in water. His chief concern safety of everyone but himself.

"We feel this illustrates the spirit which animated the whole of our staff who have been lost in this unfortunate disaster and the majority of whom were below doing their duty."

The telegram went on to list the names of the guarantee group men who were lost with the ship. Interestingly, it listed apprentice fitter Alfred Cunningham as saved, but a handwritten correction in red ink dated April 26 noted that he had now been reported missing.

"This is a very, very significant account of Thomas Andrews's heroic behaviour as the ship was going down," Mr Aldridge said.

"It's a very important historical piece of primary evidence relating to Harland and Wolff and the Titanic. It must have been sent by someone very significant and official on the other side of the pond.

"Once again it emphasises what Mr Andrews did as the ship was going down."