Help For Heroes is claiming British military veterans are waiting four years on average to seek help with mental health problems.
The charity says 30% of those suffering from the mental health impact of war have never asked for support.
The survey findings have been published ahead of the launch of a new 'Cut The Clock ' campaign to try to combat the stigma surrounding mental health issues.
The campaign coincides with a new support programme by Veterans' Gateway, a service funded by the Ministry of Defence, to help vulnerable ex-servicemen and women.
Karen Mead, head of psychological well-being at Help for Heroes, said: "Veterans are not accessing mental health support when they need it and we believe this needs to change." She added:
"Our campaign is asking the nation to call time on stigma and to let those who have served their country know it's okay to ask for help."
The charity's campaign will be launched by England Rugby World Cup winner Matt Dawson on Monday as buildings across London host a projected #StigmaClock to encourage donations and support.
Mark Beckham, a veteran who served with the Royal Anglian Regiment in Kosovo in 1999, said he went 16 years without help for his mental health. He says he'd encourage others to get help as soon as they can:
" In the military you've got your pride and you don't want to be seen as a weak individual. That's why a lot of the guys don't seek help - they don't want to be seen as a weak link in the chain."
Andrew Taylor, a veteran who served in the Royal Army Medical Corps, said he waited four years to seek help after he was medically discharged in 2013 after suffering serious injuries in a bomb blast in Afghanistan. He said:
"I lost my sense of identity, my career, the friendships I'd made and the excitement that my job had given me."
Help for Heroes' survey of 189 veterans found many did not seek help due to believing civilian services would not understand or support them (28%) and a fear of being treated differently by friends (25%) and family (19%).
It was published as Veterans' Gateway begins trialling an outreach service to phone ex-service personnel who have previously contacted its helpline.
The scheme, paid for with funding from the MoD, is based on the US Marine Corps veterans service, which makes six proactive calls for every one it receives.
Mark Collins, assistant director of Veterans' Gateway, said: "We will be monitoring the outcome to see how this trial impacts our users,
" ...but hopefully it'll mean those most vulnerable will be able to access help from Veterans' Gateway supporting organisations, on their journey to getting the right help."