The family of a bank executive who killed herself at a psychiatric unit after suffering post-natal depression is set for a “substantial” pay-out after a hospital admitted failings in her care.

Diana Mager, 33, had attempted to throw herself from the fifth floor members’ bar at Tate Modern but was pulled back from the edge by a member of the public.

She later succeeded in taking her own life, nine months after giving birth to daughter Sofia, while being treated at Green Parks House psychiatric unit in Orpington, on August 8 last year.

Oxleas NHS trust, which runs the unit, has now admitted liability for her death after months of refusing to acknowledge that it could and should have been prevented.

Her husband Adam said: “I always believed that Diana would be alive today if she had received the proper care she desperately needed. Instead, I have to explain to our daughter why her mother is no longer with us.”

His lawyer Mark Bowman, a partner at Field Fisher Waterhouse, said the trust finally admitted liability after a civil action was launched.

He said: “I am however concerned that the admission comes after the trust had initially claimed, following a full and through investigation, that Diana’s death could not have been predicted or prevented.

“There needs to be far greater transparency and honesty in mental health care in this country if the public are to have confidence in the system, especially at a time when the mental health service is reported to be in crisis.”

Mager, a risk manager at HSBC, had a history of depression. She recovered but the illness returned after the birth of her daughter in November 2011 and she began to self-harm.

She was admitted to Green Parks House, in the grounds of Princess Royal hospital, on June 26 last year.

Supervision was not raised to the highest level after the incident at  Tate Modern.

She managed to smuggle a knife into the unit and twice attempted to get large quantities of prescription drugs. Her medical record warned: “Diana is at very high risk of killing herself.”

Oxleas said in a statement: “We are very sorry and deeply saddened by Mager’s death and would once again like to offer our sincere condolences to her husband and family.

“We will now review our internal inquiry to identify further learning and ensure that this is put in place. Patient care is our top priority and we want to be open with our patients and their families.”