People with mental health problems face huge premiums or no access to travel insurance - even when their issues are well managed, a charity says.
Mystery shopping discovered four-fold increases in the cost of cover for those who disclosed mental health issues that have been stable for years.
The Money and Mental Health Policy Institute wants regulators to review the fairness of travel insurance.
Brokers say specialists can guide people to cheaper, suitable cover.
Ben Rathe had a diagnosis of depression six years ago. He was given anti-depressants by his doctor and went on a course of therapy.
The 30-year-old said that his issues had not affected him for a long time, and had no impact on his day-to-day life.
However, he was still required to declare this historic issue by ticking a box on any travel insurance application - something which he said automatically resulted in a rise of about 50% in his quoted premium.
"The lack of transparency annoys me. Insurers need to be more open with the reasons why they think I am more risky," he said.
"My condition was on the mild end of the spectrum, and has zero effect on my life, and it still has an effect on my travel insurance. I am not surprised that there is a more widespread issue here."
Other cases seen by the Money and Mental Health Policy Institute (MMHPI) included people with bipolar disorder who found that the cost of insurance rose from about £80 to hundreds of pounds after they declared their condition. Some insurers refused to provide travel cover.
'Take a closer look'
As a result of this, Helen Undy, director of the MMHPI, said that the charity's survey suggested that nearly half of those with mental health issues said they never disclosed their condition - risking their travel policy becoming invalid.
"Half of us will experience a mental health problem at some point in our lives, which could have a long-term impact on our access to insurance. If the mainstream travel insurance market doesn't work for half of customers, then it's really not working at all," she said.
"The way insurance companies calculate risk, and set their prices, is determined behind closed doors. Only the regulator has the data needed to check if it's truly fair."
A review published in June by the City regulator, the Financial Conduct Authority (FCA), found people with previous medical conditions - particularly cancer - were struggling to find affordable travel insurance, even long after treatment was finished.
An FCA spokesman said: "We are aware of the challenges people find when trying to access specialist travel insurance at a reasonable cost. This is why we have looked closely at this area to see what could make it easier for people to find the right policy.
"During our research we found there is a significant market of specialist travel insurance providers who may be more cost effective, but these can be difficult for people to find. We are working with insurance firms and charities to help provide better information which can act as signposts so that people can get access to polices more suitable for their specific needs and at more a competitive rate."
The British Insurance Brokers' Association (BIBA) has a "Find a Broker" service which allows people to search for insurance brokers in their area who specialise in finding cover for those with specific needs, such as medical conditions, mental health issues, or old age.
A spokeswoman for the Association of British Insurers (ABI) said: "Travel insurance is widely available for people with a range of long-term and serious conditions, including mental health conditions.
"As with pre-existing physical conditions, some insurers may consider them a risk factor and this can push up premiums because of the potentially high cost of treatment that is needed overseas. We are already working with the FCA and other industry and consumer bodies on how to improve signposting for customers with pre-existing medical conditions."