They were the world's oldest living conjoined twins at the time of their death in 2003 aged 53.

Snatched from their Russian mother following their birth, Masha and Dasha Krivoshlyopova were subjected to a variety of cruel medical 'experiments' by Stalin's Soviet medical authorities. 

The girls shared a blood system but had separate nervous systems - so were seen as ideal subjects for research. 

Treated as medical guinea pigs they were burnt, frozen, starved, electrocuted, kept forcibly awake and injected with radioactive and other toxic substances in the name of 'science'.

The twins were institutionalised for much of their lives but their horrific story has now been revealed in full for the first time by journalist Juliet Butler.

Ms Butler befriended Masha and Dasha and said despite sharing the same genetics, horrific childhood and all of their lives the same lower body - the sisters had wildly different personalities. 

One was a cruel, domineering 'psychopath' who was 'emotionally abusive' to her caring, empath sister who remained gentle and kind and longed for a normal life. 'Dasha was in an emotionally abusive relationship - similar to the situation some people find themselves in with a partner. 

'But while those people have a chance to leave, Dasha physically couldn't.

'Masha denied Dasha everything she ever longed for – a chance of love, a relationship with their mother, a job and even what she wanted most: a separate body.' 

Ms Butler, who has written a novel based on their lives, The Less You Know The Sounder You Sleep, has provided fascinating insights into the tragic pair that reignite the nature versus nurture debate. Used as guinea pigs 

Almost immediately after the twins were born on January 1950 by caesarean section, they snatched from their mother Yekaterina - who was told her babies had died shortly after birth.

Dasha and Masha were taken away to, a medical institute in the Moscow region, the Academy of Medical Sciences Pediatric Institute, to be used as medical guinea pigs.

The girls shared a blood system but had separate nervous systems - so were seen as ideal subjects for research.