A woman has pleaded guilty mid-trial to the murder of her boyfriend, who she buried in their back garden. Fiona Beal, 50, was on trial at the Old Bailey in London for the murder of Nicholas Billingham, 42, whose partly mummified remains were discovered in March 2022 four-and-a-half months after he was last seen. Beal, a primary school teacher, was accused of tying Billingham up and stabbing him to death “in cold blood” before burying his body in their garden in Northampton. She previously pleaded guilty to the lesser offence of manslaughter by reason of a loss of control, but denied murdering Billingham between 30 October and 10 November 2021. After changing her plea on Friday, partway through the trial, she will be sentenced next month. Beal was arrested in March 2022 after police discovered the body and her actions were uncovered by police through entries in her diary. The prosecutor Hugh Davies KC told jurors that Beal, described as “a high-functioning professional”, had messaged several people on 1 November 2021 to say she and her partner had contracted Covid-19 and were isolating at home. There was no evidence Beal took a Covid test, the prosecution said. Similar messages were sent from Billingham’s phone on 2 November, which was alleged to be Beal “pretending to be him” in a move that was “as heartless as it was self-serving”. Beal then told friends and family that she and Billingham had split up, with one message saying he had left her because he was having an affair with another woman. The prosecution said this was “completely false” but Billingham appeared to have cheated on Beal previously. Beal returned to work as a year 6 teacher and was able to “fully discharge her considerable responsibilities” before her mental state deteriorated in February 2022, the court heard. In March she rented a cabin in Cumbria and sent concerning messages to family members that prompted them to call the police to check on her, the prosecutor said. In the cabin, police found journals “written in her hand” that contained “some unambiguously clear declarations of what she had done”, Davies said. “She had planned to, and had, killed him in cold blood. She had purchased a forged handled utility knife in the days before. She had a chisel and cable ties. “Promising sex after a bath, she stabbed him in the neck when he was wearing a sleep mask and was probably cable-tied on their bed.” One of Beal’s journal entries read: “It was harder than I thought it would be. Hiding a body was bad. Moving a body is much more difficult than it looks on TV.” The court heard that Beal had broken the bannister rails in the upstairs of the house in order to move Billingham, who was 5ft 11in and weighed 14 stone (90kg). The journal entries triggered a police investigation, which soon established that Billingham had not been seen or spoken to since the afternoon of 1 November 2021. Davies said the journal entries also suggested Beal had been “controlled and manipulated in the relationship” and detailed “unpleasant things” Billingham had done in explaining why she killed him. She also had a split personality, or alter ego, who she called Tulip 22, “who is capable of wholly different and darker conduct than her public persona of a committed teacher”, the prosecution said. Beal will be sentenced on 29 and 30 May.