A former soldier has been found guilty of murdering his grandmother by smothering her with a pillow after his mother gave the elderly woman a cocktail of crushed tablets and whisky.

Barry Rogers, 33, and his mother Penelope John, 50, both face life sentences for the murder of Betty Guy, 84, at her home in Pembrokeshire, south-west Wales.

Details of the plot emerged five years after the murder when a former girlfriend of Rogers told police that he had admitted killing his grandmother in 2011.

Police arrested him and his mother in 2016, installed listening devices in her home and heard the pair discussing the killing.

A jury at Swansea crown court heard that after Guy’s death, John phoned 999 to say her mother had been suffering with bowel and stomach cancer and had died in bed.

But medical records showed Guy was never diagnosed with cancer, and a doctor whom John claimed had told her she was terminally ill said the conversation had never taken place.

No postmortem was carried out on Guy, and her daughter had her body cremated days after her death.

John told the jury her relationship with her mother had been “amazing” and that her ashes had been scattered in her garden which she visited every day. She said she had inherited only a washing machine and a tumble dryer and the recorded confession had been a “joke”.

Asked about a mercy-killing pact, she said: “I would never agree to that. My mother was my life.”

In the secret recording John was heard telling her son that she had been arrested for murder, and Rogers replied: “But I did it.”

Rogers was also heard saying: “No, honestly, you have got nothing to worry about, it’s me that’s done the act … There’s nothing to worry about. Nothing to worry about. Keep our story the same.”

Paul Lewis, prosecuting, claimed the exchanges showed the mother and son had plotted to murder the elderly woman.

Outside court, Gemma Vincent, a senior crown prosecutor in CPS Cymru-Wales, said: “Covert recordings such as these are not common. In this case it provided compelling evidence for the CPS to present to the jury, demonstrating that the killing of Betty Guy was deliberate and planned. Ms Guy’s life was cut short by her own daughter and grandson – our thoughts and sympathies are with those affected.”

Guy’s daughter Lorraine Matthews said her family had been “shocked and horrified” to learn the truth about her death. In a statement released through Dyfed-Powys police, she said: “Seven years after my mother’s death, my brothers, my sons, myself and other members of the family, were shocked and horrified to learn from the police that my mother, Betty Guy, may not have died from natural causes and that my sister and her son were to be charged with her murder. Now that the case has drawn to a close we are satisfied that justice has prevailed and now we can close this very sad chapter in our lives.”

The pair will be sentenced on Thursday.