The chief executive of the East of England Ambulance Service NHS Trust (EEAST) has stepped down due to ill health.
Dorothy Hosein had been off since October and told staff her illnesses included a "severe bout of Covid-19".
The trust has been in special measures since October and is facing allegations of bullying.
She said: "It is with great sadness that I have decided to relinquish my post to focus on my wellbeing."
In a message to staff, Ms Hosein said she had not fully recovered from coronavirus and had decided to leave in order to do that.
Ms Hosein joined the trust as interim chief executive in 2018, and was appointed to the role permanently in 2019.
Ms Hosein said staff were "are at the heart of this service" and it had been a pleasure working with them.
"I am hugely proud and thankful for the work we have done together to improve patient safety and experience," she said.
EEAST chairwoman Nicola Scrivings said "significant improvements" to performance had been made under Mrs Hosein's leadership.
"The trust moved from being the worst-performing ambulance trust in England to one that regularly met and even outperformed national standards," she said.
"Staff recruitment and finances also improved, and new approaches to leadership meant the trust was as well-positioned as possible to meet the challenges of Covid-19."
The trust's medical director and deputy chief executive, Dr Tom Davis, will continue as interim chief executive.
A permanent replacement is expected to be appointed in April, the trust said.