Lucy Letby was handed a whole life order without the possibility of parole after being convicted of killing seven vulnerable newborns and attempting to kill six more.
Having a key to her own cell means that Letby, 34, can lock herself away from fellow inmates if she feels threatened, and is believed to be a reward for good behaviour.
A family member of one of the victim’s said: “We thought they were throwing away the key, and now it turns out she has her own.”
Letby, who carried out the murders at the Countess of Chester Hospital between 2015 and 2016, is now being held at the privately owned Category A HMP Bronzefield in Ashford, Surrey.
She was previously detained at HMP Low Newton in Durham, which has a reputation for housing some of the UK’s worst female killers.
A prison insider told The Mirror: “It means if she wants to socialise she can, and if she wants privacy she can have that too. It’s really no different to living in a hotel – but you’re surrounded by criminals.”
It was previously revealed that Letby had befriended fellow baby killer Michelle Smith, who had poisoned her daughter in 2007, and Shauna Hoare, who had helped kiled 16-year-old schoolgirl Becky Watts.
A source told The Mirror that the trio of killers had been playing board games together and sat with each other when it came to eating meals.
“They were together 24 hours a day – they were a well-known threesome and were literally never apart,” they said.
“They’d spend hours sitting on the wing landing playing board games, eating dinner together and tanning in the courtyard.
“I think they got on because they all believed they were innocent.”
In August, Letby refused to attend her sentencing when Mr Justice Goss handed her 14 whole life terms, while the prosecution has decided to pursue a retrial in the case of Baby K, a girl born in February 2016.
A month later, Letby launched an appeal against all of her convictions, while a public inquiry into her offences was formally launched in November.