The 86-year-old was found dead in her bungalow in Magazine Lane, Wisbech, at 10.45am on Sunday - but details of the horrific attack have only been released this afternoon.

Her death had originally been treated as unexplained but a post-mortem revealed she died as a result of stab wounds to her neck and chest.

The pensioner’s clothing had also been set alight, however the fire did not spread to the rest of the house and self-extinguished.

Det Chief Insp Jon Hutchinson said: “This was a horrific murder of a defenceless elderly woman in her own home.

“She was stabbed several times then set on fire in an apparent attempt by her killer to hide her injuries and destroy evidence.

“The fire damage meant it was difficult to initially establish the cause of Mrs Crown’s death. However the stab wounds were revealed during the post-mortem on Tuesday evening.

“I would appeal to anyone who thinks they may know who is responsible for this brutal murder.

“Did a friend or loved one come home on Saturday night or Sunday afternoon smelling of smoke?

“I would urge you to do the right thing and call police to ensure the killer is caught and brought to justice.”

Mrs Crown had last been seen alive on Friday, but she spoke to a friend on the telephone at about 5pm on Saturday.

The bungalow was cordoned off this week as forensic officers combed the property. Detectives searched drains, wheelie bins and hedges in the freezing fog outside and house-to-house enquiries are continuing.

Mrs Crown’s husband, Jack Roland Crown, who was known as Ron or Ral, died on Boxing Day, 2009, at the North Cambs Hospital, Wisbech.

He and Una had been married for 60 years.

At his funeral, donations for the Trafford Ward at the hospital totalled £354.

Fifty detectives are involved in the murder hunt and Det Chief Insp Hutchinson said police would use “every tactic available” to bring the killer to justice.

He said: “We know Mrs Crown was last seen by her family at 11am on Friday, and we believe she was spoken to by a friend on the telephone on Saturday.

“So we have narrowed her time of death down, to between 6pm on Saturday and 10.45am on Sunday morning, when she was discovered by family members when they came to check on her.

“Understandably the family is traumatised and is receiving support from specialist police officers.

“The forensic investigation is ongoing and a post mortem on Tuesday at Peterborough City Hospital identified that she was murdered and this was the result of a brutal assault.

“We have launched a major investigation with up to 50 detectives involved from the Bedfordshire, Cambridgeshire and Hertfordshire Major Crime Unit, working closely alongside local officers from the Safer Neighbourhood Team.

“We are doing a number of further searches and checks at the property to find out if there was a break-in or forced entry, that is one line of investigation we are looking at.

“It is early days and we are piecing together the jigsaw to discover what happened. A cordon remains around the house and officers will be visible conducting inquiries and providing reassurance to the local community.”

• Anyone with any information should call the Major Crime Unit on 101 or Crimestoppers, anonymously, on 0800 555111.

• An inquest into the death of Una Vivian Crown is due to be opened by Coroner William Morris tomorrow at 10am.