The failure to address the mental-health needs of people with HIV could lead to an increase in infections, a cross-party group of MPs suggests.
People with HIV are twice as likely to experience mental-health difficulties.
And in those with depression, support raises adherence to medication by 83%.
But most HIV clinics have no mental-health professionals on staff, which, the MPs say, could be reversing progress made over the past decade toward ending the epidemic in the UK.
The All-Party Parliamentary Group (APPG) on HIV and AIDS met with patients living with HIV at a range of hospital trusts throughout England, as well as numerous healthcare professionals.
And unless serious mental-health treatment shortfalls are addressed, the government will fail to achieve its target of zero transmissions by 2030, its report says.
Becky, from Sheffield, who only shared her first name, told BBC News although she had been offered a psychologist on diagnosis, she still felt isolated.
"There needs to be funding in all areas of the country for support and, in particular, peer support," she said.
"There is a bit of a postcode lottery at the moment when it comes to getting it.
"All I wanted was to meet other people living with HIV.
"However, at the time this was not offered to me as it was not a thing in my area.
"While people like me are medically well, mental health is still suffering and that's down to the lack of support that's available."
There have been significant developments in the clinical treatment of HIV in the UK since the 1980s.
Those living with the disease now have a life expectancy comparable to those who do not.
Much of the stigma exists because people do not realise it is impossible for patients with an undetectable viral load to pass on the virus.
Russell Hayward, 29, from Manchester, who was diagnosed in September 2019, told BBC News that "the fight is not over".
"It does seem like once you have your medication and you're on the path to being undetectable, you do feel a bit abandoned," he said.
"The fight is far from over. One person's stigma is a stigma too big. We must get louder. I will keep telling people until my last dying breath if I have to. I won't give in."
HIV support services are there for those who are newly diagnosed or struggling to accept their status.
But, the MPs found, local funding cuts meant some people living with HIV were relying on generic mental-health services inadequate for their needs.
Andrew, from London, who was struggling with addiction after being sexually assaulted, told BBC News the help he had been offered had not been fit for purpose and he had had to look elsewhere.
"When I did get to a therapist, she didn't specialise in addiction and sexual assault survivors," he said.
"Then, there were some options for community-led group stuff but I wouldn't have felt comfortable with that."
Andrew, who only shared his first name, also said the NHS's recent shift of focus to HIV prevention had made him feel more stigmatised than ever.
"Some of the Prep messaging feels as though it has regressed to, 'Take this and, thank God, you won't get HIV - as that is awful,'" he said.
Pre-exposure prophylaxis (Prep) is a daily tablet that stops people contracting HIV.
Prep is currently freely available for high-risk patients in Scotland and Wales, but many in England have had to wait until they can gain access to an impact trial that first began in September 2017.
And as of October 2019, at least 15 people in England had tested HIV positive while waiting for a place on the trial.
Stephen Doughty, who chairs the APPG on HIV and Aids, said there needed to be a "greater understanding" of living with a stigmatised health condition.
"This should be reflected in a new national HIV strategy," he said.
"We heard some really disturbing evidence about patients who have committed suicide because of stigma and the traumatic process of navigating the benefits system while grappling with HIV and mental health issues," he added.
The report highlights the case of a woman who, having received employment and support allowance for many years, killed herself after being ruled "fit for work", in 2013.
It was only after her death that the decision was overturned.
Mind chief executive Paul Farmer said: "It is only by developing a cross-government approach to mental health will we be able to really improve the experiences of people with mental-health problems.
"We hope this report goes some way towards helping to make that case."