Almost 2,000 people waited more than three months for a second mental health treatment in Manchester last year, figures show.
The findings come as a leading staff body for the profession says it is "unacceptable" so many are suffering delays for vital treatment across England.
The NHS' talking therapy service treats patients for common mental health problems like stress, anxiety and depression.
In the year to November 2022, around 9,240 patients received a second treatment in the former NHS Manchester CCG area – but 1,975 (21.4%) had to wait more than 90 days from their first one.
This was a rise from 9.1% in the year to November 2019.
The Royal College of Psychiatrists, a staff body, said delaying treatment could "put patients' wellbeing at risk".
Dr Adrian James, the organisation's president, said: “Some patients' mental health may deteriorate if left untreated, which can have a detrimental impact on their quality of life and have potentially life-altering consequences.
“There is also a danger that long waiting lists will undermine people’s faith in mental health services and discourage them from seeking the support they need.”
He added staff are "working as hard as they can" but are struggling with limited resources and problems with retention and recruitment.
Across England patients are facing lengthy waits for mental health treatment.
The figures show 154,000 people (22.5%) were waiting more than three months between their first and second treatment in the year to November 2022, up from 120,000 (19.2%) over the same period in 2018-19.
Mark Rowland, chief executive of the Mental Health Foundation charity, said these statistics should come as "no surprise" given the "huge pressures" mental health services are facing.
He urged the Government to prioritise preventative approaches – focused on tackling mental health problems before professional help is needed.
Standards set by the National Institute for Health and Care Excellence state 75% of patients should get a first treatment within six weeks.
The figures show services in the former NHS Manchester CCG area were meeting that target in the year to November – with 83% of 24,670 patients receiving treatment within six weeks.
As of November, around 4,315 people were waiting for a first treatment in Manchester – up from 4,245 in November 2019.
An NHS England spokesperson said: “The NHS is committed to increasing support for patients through its world-leading talking therapy services, and in November there were 60,000 more appointments completed compared to the previous month.
“The NHS is also increasing the number of trainee places for more people to join the workforce, so if you are struggling with your mental health, you can either refer yourself to your local NHS talking therapies service online or contact your GP,” they added.
The Department of Health and Social Care said it was already taking a preventative approach to tackling mental health issues and expanding access to the talking therapies service.