Media playback is unsupported on your device Media caption James Tringham, who has bipolar disorder, explains that being at work has a "normalising effect"

Hayley Smith says her manager encouraged her to apply for other jobs when she told her she had depression.

The news about her illness spread across the office. "It was horrible - I felt really exposed," she says. After a few anxious, unhappy months, she left.

Hayley is one of up to 300,000 people with mental health problems who leave their jobs each year, a report says.

The report by mental health experts also says poor mental health costs the UK economy up to £99bn each year.

Paul Farmer, co-author of the Thriving At Work report, said mental health was a taboo subject in many workplaces.

Mr Farmer, who is chief executive of mental health charity Mind, said: "Opportunities are missed to prevent poor mental health and ensure that employees who may be struggling get the support they need.

"In many instances, employers simply don't understand the crucial role they can play, or know where to go for advice and support."

The review makes 40 recommendations for how to support employees to remain at work, including creating an online wellbeing portal and using digital technology to support workers in the gig economy.

Companies are also being encouraged to include a section on employee mental health in their annual reports. Currently only 11% of companies do this, the report said.

Prime Minister Theresa May, who commissioned the report, said it showed "we need to take action". The civil service has agreed to take on the report's recommendations.

Image copyright Hayley Smith Image caption Hayley Smith lost her job when she told her employer about her depression

One employer the report praised is the insurer, Aviva.

James Tringham, who has worked for the company for seven years, said it helped him get his life back on track after suffering with bipolar disorder.

"In 2009, I'd reached crisis point with my mental health, and I gave up my career as a solicitor," he says.

"I was unable to work for over a year - the prospect of returning to work was just so frightening and not something that I could imagine."

Eventually he got a job in the insurer's contact centre. "They gave me a way back into employment and I have worked my way back up.

"Aviva have been brilliant in supporting me to manage my mental health at work, and have given me my confidence back.

"There's an understanding, which means little things like adjusting my workload when I'm feeling on the low side can really help. And the staff - both officially and unofficially - form a great support network."

Image copyright Getty Images Image caption Footballer Aaron Lennon returned to training after receiving mental health support

The review says employers should:

Create a mental health at work plan

Build mental health awareness by making information and support accessible

Encourage open conversations

Provide good working conditions and ensure employees have a healthy work-life balance

Promote effective people management, with line managers holding regular conversations about health and well-being with their staff

Routinely monitor employee mental health

The review said that people with long-term mental health problems were leaving jobs at twice the rate of colleagues with no such issues, although it did say that some people may be counted twice - if they left one job, returned to work elsewhere after a break but were then unable to continue in their new post.

The report also pointed out the cost to employers, estimating they were losing £42bn each year because of staff suffering from mental health problems.

Mrs May said: "It is only by making this an everyday concern for everyone that we change the way we see mental illness, so that striving to improve your mental health - whether at work or at home - is seen as just as positive as improving our physical well-being."

What is mental health?

For Hayley Smith, support from her employer could have made her chronic depression more bearable.

After leaving her job, she started her own company. Five years on, she now employs others.

"In the long term, it's given me awareness of mental health," she says. "And the awareness to make sure I was never going to make people feel the way I was made to feel."