Young people aged 18-24 are also most likely to suffer psychological problems but least likely to seek professional help.
Work is named as the most likely cause, followed by family, health and finance.
The poll, part of our crusade for better mental health, reveals the shocking extent of the problem facing our nation.
It comes as Prime Minister Theresa May spoke to the Sunday Express about the “stress” of political life and praised Scottish Tory leader Ruth Davidson after she recently revealed she had suffered a mental health breakdown.
Ms Davidson also confessed she felt she could never be a candidate to be prime minister as a result.
Mrs May said: “In politics, every week brings its ups and downs – every walk of life has its stressful moments. To me, in doing this job, the important thing is to remember that it’s a huge privilege.”
Our survey, conducted last week, involved 1,000 adults aged 18 and over.
It found 49 per cent of people had suffered from mental health issues such as depression or stress.
Women were the worst affected at 55 per cent. Of that 49 per cent affected, 78 per cent said they had sought professional help.
Those aged 18-24 were least likely to seek help (67 per cent), while those aged 65-plus were most likely (89 per cent).
Worryingly, 18-24 year-olds said they are turning to alcohol as a way to ease their problems, with 20 per cent saying they consider drinking to be a useful way of coping.
Work/ Family balance
The most common catalyst for mental health issues is work – cited by 51 per cent – closely followed by family (45 per cent), health (37) and finance (33).
Social media was named as the biggest problem for 25 per cent of 18-24s. It is commonly believed that a quarter of the population has suffered a mental health problem.
Our findings suggest the true number is far higher.
Andy Bell, deputy chief executive of mental health research charity the Centre for Mental Health, said: “These findings are very important and show that having a mental health difficulty affects at least half of us – every family, every school and every workplace will be affected.
“It also reflects the fact that we need to do every thing we can to protect mental health from childhood onwards, as this can have lifetime benefits.”
Mr Bell added: “Despite its prevalence there is still a long way to go before the NHS treats mental health on an equal basis to physical health and, despite recent investment, services are still patchy.
Having a mental health problem can do a lot of harm to wellbeing, physical health and relationships.”
Stephen Buckley, head of information at mental health charity Mind, said: “This research highlights a high prevalence of stress and poor mental health in the UK.”
And he added it was worrying to learn around seven out of 10 18-24 year olds had experienced a problem or stress.
He said: “There are lots of factors that could impact on the mental health within this age group.”
The high number of people citing work as a cause of their distress has alarmed the charity Mental Health First Aid England.
It is calling for a change in the law to give mental health equal importance to physical health in the workplace.
Jaan Madan, workplace lead for the charity, said: “It’s encouraging that young people feel able to come forward to seek support with their mental health but concerning that so many are struggling with conditions such as depression and anxiety.
“With the pressures on young people entering today’s competitive job market, from heavy workloads to high student debt, a supportive workplace is perhaps more important than ever.”
A Workplace Wellbeing Index survey of 44,000 staff across 74 employers in England and Wales, conducted by Mind, reported that 48 per cent of workers had experienced poor mental health but only half of them had told their employer about it.
Emma Mamo, head of workplace wellbeing at Mind, said: “Most of us will experience high pressure in our jobs from time to time. In small doses, it is not necessarily problematic and can actually be helpful, for example when it comes to meeting deadlines.
“It’s only when pressure becomes prolonged or unmanageable stress that it can negatively affect our physical or mental health problems.
“It’s really important that employers take steps to help tackle the causes of work-related stress and poor mental health but, too often, they don’t know where to start.”
I said if I go home I will kill myself’
SARAH LAMB first suffered from depression when she was just 13.
By the time she was 14 she had begun to self harm.
Sarah, now 29, and from Chelmsford, Essex, said: “I was very stressed at school. I was a perfectionist and felt I was failing at everything.
“I had seen a GP but he didn’t do anything.
He decided to “watch and wait” and told me to place an elastic band over my wrist and ping that instead of cutting myself.
“The cuts were like seeing the physical hurt in my head and it released something. I was pretty much left to my own devices until it got so bad I tried to kill myself.”
Sarah had been taken to A&E with self-inflicted injuries on at least 12 occasions in three months before she was admitted to hospital in December 2006.
When she was 17, Sarah, begged to be hospitalised, fearing suicide.
“I said if I go home I know I am going to kill myself,” she added.
But there was no place on a children’s ward and she was sent to an adult institution with dangerous patients who assaulted her.
Outlining the problems facing anyone desperate for NHS help, Sarah said: “Waiting times need to be reduced, and often you only get six therapy sessions. No significant progress can be made in six weeks.
“I definitely think that early intervention is key. I would not have tried to kill myself at 17 if I had had help beforehand.”
Another who suffered from mental illness is Regie Butler, 50, a police sergeant.
He had suicidal thoughts when his marriage collapsed and he faced financial ruin.
“I could not see any way out, and I felt like a complete failure. It was not a case of wanting to die, but more no longer wanting to live,” he says.
“Back then there was a real stigma, a ‘big boys don’t cry’ attitude; so I buried it and carried on.”
Five years ago the suicidal thoughts returned and Regie, from Devon, found professional help.
“Do not be afraid to say, ‘I am feeling rubbish. I am having thoughts of suicide. I need a little help’. Things can always get better.”
PM praises Ruth Davidson’s ‘incredible bravery’
THERESA May praised Scottish Conservative leader Ruth Davidson for coming forward to talk publicly about her battle with mental illness.
The Prime Minister said: “I thought Ruth was incredibly brave to give that interview. I think it will give hope and encouragement to an awful lot of people to see a senior politician being so open and so honest about her own mental health and what she’s been through.”
Davidson, who is pregnant with her first child, has said she was diagnosed with clinical depression when she was 18.
She said she had started self-harming at 17, punching walls, and cutting her stomach and arms with blades or broken glass.
She has ruled out ever being leader of the Conservatives, despite frequently being tipped for the position.
She said: “No. I value my relationship and my mental health too much for it. I will not be a candidate.”
In our poll more than half said that well-known people like Ruth Davidson, and also Prince Harry, who speak about their mental health issues, has helped reduce the stigma surrounding the issue.
Prince Harry revealed last year that he sought counselling after enduring two years of “total chaos” while still struggling to come to terms with the death of his mother, Diana Princess of Wales.
It’s acceptable now to admit it’
John Woodcock, independent MP for Barrow and Furness, announced he was suffering depression in December 2013.
He has been prescribed antidepressants “to relieve black moods”.
The 39-year-old father-of-two, who has been an MP since 2010, said: “Every time someone in the public eye speaks about mental health issues it helps to make it seem more normal.
“Things have changed lots in the few years since I talked about suffering from depression. Back then it was a big deal and I ended up talking about it on Newsnight with Jeremy Paxman. Thanks to campaign groups like Mind, it has been more acceptable and normal to admit it but there still remains a massive problem over services that people can access.”
He added: “The Government and NHS is genuine in its desire to create parity of physical and mental health but we are miles away from it.
“In my constituency I see many people who can’t get the help they desperately need.
“The youth mental health system is overrun and cannot offer the support until people are absolutely desperate, which is the opposite of how it needs to be as early prevention is so important.”