A housing group said it is "devastated" and "sorry for our part" in failing to realise one of its tenants had been left dead in her flat for two and a half years.
Sheila Seleoane, 58, was last heard from in August 2019, the last month she made a rent payment.
When police forced their way into her flat in Peckham in February 2022 - after neighbours noticed a balcony door swinging open following Storm Eunice - her body had to be identified by dental records.
Residents had reported the presence of maggots and flies to Peabody Group "within weeks" of the August date she had last been confirmed alive, but the housing association closed the case the month after.
"We didn't ask the most fundamental question - is Sheila ok?" said Peabody's chief executive, Ian McDermott.
"I am so sorry this happened," he said. "We've apologised to the family. We're deeply sorry for what happened.
"The biggest apology though I think does go to the residents of Lord's Court. They did tell us that something was wrong."
Six months after Ms Seleoane made her last rent payment, Peabody made an application for direct payment of Universal Credit.
The housing association has not repaid this money but has pledged to do so.
According to an independent investigation commissioned by Peabody, the COVID-19 lockdown "exacerbated the length of time the body remained undiscovered, but was not the cause of the delay".
Peabody said that it had recorded 89 attempts to contact Ms Seleoane, but recognised these were not substantive and none were successful.
By October 2020 the housing association had contacted the Metropolitan Police to perform a welfare check on Ms Seleoane and an officer incorrectly told them she was safe and well.
The force said that the staff member had since left the force but would have faced a professional standards enquiry if they had still been employed.
A coroner's inquest this week found that the police and Peabody had missed numerous opportunities to discover her body.
Dr Julian Morris delivered an open verdict and said: "To lie undetected for in all likelihood over two years is difficult to fathom in 2022."