Some prisoners who were deprived of the right to correspondence, hid bottles with their letters under one of the theatre's pillars. Picture: Nizhny Tagil State Drama Theatre

Signed by three convicts abused as slaves, their heartfelt message to posterity was buried in 1954 amid the bricks and mortar of Nizhny Tagil State Drama Theatre, one year after Stalin died, but before reforms of his vast prison system had taken root.

It read: 'This message was immured on 15 March 1954. There was no cheers from crowds and no orchestra as it was happening.

'But it will tell our descendants that this theatre was built not with the forces of Komsomol (Young Communist) brigades - as they will be claiming - but on the blood and bones of prisoners, the slaves of the 20th century.

'Hello, future generation! And may your era have no slavery and no humiliation of man by man.

'Cheers from us,

'Prisoners I.L.Kozhin, P.G.Sharipov, U.N.Nigmatulin'.

The fate of the three men who left their message for the future is unknown. Picture: Nizhny Tagil State Drama Theatre

Written on a sheet of iron with red-brownish nail polish, the message was dated 15 March 1954, and was found just over 50 years later during renovations of the building, though its discovery was not widely publicised.

Indeed, there maybe more messages from Stalin's prisoners who worked on the theatre, with claims that more letters, in a bottle, are buried under one of the pillars.

The theatre website said: 'This letter lay for half a century, immured inside the second floor ceiling above an alcove. It was discovered accidentally during the renovation in March 2005.

'Locals knew that TagilLag prisoner labour was used for hardest parts of construction of the theatre. Old-timers say that prisoners were brought in carts at dawn and taken back towards village of Kirpishniy and others at dusk.

'Some brave locals threw sacks with bread and potato across a fence surrounding the building site.'

Lev Samuilovich Libenshtein, who oversaw construction works at Theatre Square, said some prisoners who were deprived of the right to correspondence, hid bottles with their letters under one of the theatre's pillars. No-one knows what is written there.'

Nor is the fate known of the three brave men who left their message for the future, an act of criticism of the authorities that might have cost them their lives if found during the Soviet period.