A grubby scrap of paper with a blurry postmark on its sixpenny stamp recording the date 11 April 1912 is coming up for auction and is estimated to sell for up to £12,000.

The unique label is from a parcel addressed to the Marconi wireless operator on the Titanic, never delivered because three nights later the “unsinkable” Titanic struck the iceberg in one of the most famous maritime disasters of all time.

Among the 1,503 lives lost was the wireless operator, Jack Phillips, aged 25. He had stayed at his post, with his junior, Harold Bride – who survived – sending SOS messages until the sinking ship had lost almost all power.

The label was posted from Chelmsford and addressed c/o the White Star Line in Southampton. The package, now lost but believed to have been a bundle of blank telegram forms for use on board, was given to the first officer of the Titanic’s sister ship Olympic to take to New York. It would then have been handed on to Phillips when the Titanic reached the port at the end of its maiden voyage.

On the night of 14 April 1912, Phillips is believed to have delayed passing on a message to the bridge from another ship, warning that the Titanic was steaming straight towards an ice field, because he was working through a pile of such telegrams, relaying messages from passengers to Newfoundland.

Alec Bagot, the wireless operator on the Olympic, which carried the parcel to New York for delivery to the Titanic. Photograph: Julian Roup

The label was given as a souvenir by the Olympic officer and later acquired by a private collector, but its significance was only recently realised.

Gary Watson, an expert at Mossgreen auctioneers which will sell the label in a postal history auction in Australia later this month, regards it as one as the most evocative postal items associated with the ship.

It is being sold with photographs of the wireless equipment, and of Alec Bagot, the Marconi operator on the Olympic, the first launched of the three great sister ships, and the only one to survive for a full working life. The Britannic was requisitioned as a hospital ship in the first world war, and sunk by a mine in 1916, but the Olympic continued in service until 1935, and was broken up in 1937.