Some of Britain's biggest employers are pressing the government to honour a promise to give mental health the same status as physical health at work.
Royal Mail and WH Smith are among the companies asking the PM to follow through on her manifesto pledge to update health and safety legislation.
That would mean employers would have to provide appropriate training for staff to deal with mental ill-health.
About one in six of people at work have symptoms of a mental health condition.
A government-commissioned review put the cost of those conditions, such as depression, anxiety or stress, to the economy at between £74bn and £99bn a year.
At the last general election, the Conservatives said they would amend health and safety rules so employers would have to treat mental health the same way they treat physical health.
Some 50 executives, including Lord Sugar and bosses from Thames Water and Ford of Britain, have written to Theresa May, asking her to prioritise this particular pledge.
The companies behind the letter argue the promised change in the law would help break the stigma of mental illness at work.
Fionuala Bonnar, from Mental Health First Aid England said: "The change in legislation we are calling for will establish a baseline for protecting mental health in the workplace, ensuring no one is left behind."
Poor mental health affects half of all employees, according to a survey of 44,000 people carried out by the mental health charity Mind.
It says around 300,000 people lose their job each year due to a mental health problem.
A recent poll by the Institute of Directors found less than one in five firms offered mental health training for managers.