Three killers had their murder convictions quashed because a police officer involved in the case failed to disclose she knew one of the jurors in their trial, it has been revealed.

DC Rebecca Bryant, a family liaison officer in the case, did not disclose that her son’s girlfriend was sitting on the jury in the trial of three men accused of stabbing Lynford Brewster to death in Cardiff.

Bryant, 46, sent text messages to the juror warning her to keep quiet about knowing her and even offered her lifts to court.

One text read: “Don’t tell any of them who u r to me tho in case they think I’ve told u about it although u know I haven’t xxx.”

Bryant sat with Brewster’s family in the courtroom, close to the jury box at Cardiff crown court. The court heard father-of-three Brewster, 29, was stabbed to death on an estate in the Welsh capital in a dispute over drugs.

Dwayne Edgar, 31, Jake Whelan, 26, and Robert Lainsbury, 25, were convicted in December 2016 of murder and sentenced to life in prison.

All three had their convictions quashed after the relationship between the detective and juror was discovered.

Dwayne Edgar (right), Jake Whelan (centre) and Robert Lainsbury were convicted of murdering Lynford Brewster in Cardiff in 2016 in a second trial. Photograph: South Wales police

A court of appeal judgment said: “Several weeks after the trial an appellant’s solicitor received information that … Det Con Bryant, who had attended court during the trial to provide support to the deceased’s family, had a close relationship with a member of the jury.

“That officer’s son was in a long-term relationship with the juror. There was recovered from the officer’s phone a series of texts which the pair had exchanged at that time.”

In one message, the detective offered the juror a lift to court: “If ur on the murder ul be finished same time as me most days u can have a lift to mine afterwards if u wanted x.” The juror replied, saying: “Fab thanks! The bus is a nightmare x.”

Another read: “Remember what I sed though, as long as you don’t know any of the witnesses that’s fine ... I won’t be there hardly and I’m not a witness anyway so that ok u don’t need to worry bout that.”

The court of appeal quashed the convictions and Edgar, Whelan and Lainsbury were ordered to face a new jury.

Following a seven-week trial at Bristol crown court, the three were convicted for a second time. They will be sentenced later this month.

Assistant chief constable Jeremy Vaughan said: “When issues regarding the original trial came to light, the matter was voluntarily referred to the Independent Office for Police Conduct and has been investigated by the South Wales Police professional standards department.

“Our investigation, which has also been subject of independent review, has not found any evidence that the officer intended to undermine the criminal justice process, and following a formal submission to the Crown Prosecution Service the matter will now be dealt with through South Wales Police disciplinary processes.

“Our thoughts remain with the family of Mr Brewster who we have continued to support throughout this difficult time.”