Funding to mental health trusts has fallen in recent years, according to reports.
Budgets for trusts in England fell by 2% from 2013/14 to 2014/15, Freedom of Information requests showed, according to the BBC.
Hospital trusts' operating budgets rose by 2.6%, the charity Health Foundation said, in contrast to the Health and Social Care Act 2012's emphasis on the importance of mental health alongside physical well-being.
The Department of Health said money given to trusts did not represent all the funding for mental health care, which is also directed through local authorities, private sector organisations and hospital trusts themselves.
The BBC said that of the 53 out of 59 mental health trusts in England that responded to the Freedom of Information request, 29 said their budgets would also be lower this year than last.
In Scotland and Wales, health boards have also seen fluctuations in funding levels for mental health care.
Anita Charlesworth, chief economist at the Health Foundation, said: "Mental health hasn't increased as a share of NHS funding, despite the fact that there are huge demands on the system, and access to care for mental health still falls way below that for physical health.
"The NHS clearly sees parity of esteem as a key priority and want to prioritise improving mental health. But problems in our hospital sector means that money increasingly is getting sucked in to meet their rising costs, and the NHS is struggling to actually commit resources to fund mental health providers."
A 1.4% fall in number of nurses working in psychiatric sectors between 2010-11 and 2014-15, was also reported.
A Department of Health spokesman said: "These figures do not show the full picture for mental health spend - councils, third sector organisations and NHS England all play a role in providing services, and all receive government funding.
"We have made more money available than ever before for mental health, increasing our investment every year since 2010 to a record £11.7 billion last year."
In Wales, spending was reported to be down 1.1% in 2014-15 and up a likely 1.2% in 2015/16.
The Welsh Government said mental health spending has been ringfenced since 2008 and more was spent on it than any other part of the Welsh NHS.
A spokesman said: "The budget for mental health services in Wales has increased year-on-year and total spend this year is £600 million, up from £510 million in 2010-11."
Mental health spending in Scotland was up just 0.1% last year, and down a projected 0.4% this year, the BBC said.
Jamie Hepburn, Scottish Government Minister for Sport, Health Improvement and Mental Health, said: "Mental health spending has increased by almost 40% under this government and we have made a commitment to invest an additional £150 million over a five-year period in improving access to mental health services."
Northern Ireland was the only country to see a reported increase in both years, with 1% additional spending last year, and 2.6% more this year.