Spending in real terms on mental health has declined for the first time in a decade, a report for the Department of Health has found.

Although one of the coalition's first big policy announcements was to declare that mental health ought to have "parity with physical health in the NHS", investment in mental health for working-age adults dropped by 1%, once inflation is taken into account, to £6.63bn. For the elderly the recorded fall in real terms spending was 3.1% to £2.83bn.

In total, spending on mental health services in England dropped by £150m, the first fall since 2001. However this drop comes after a decade of rising investment: in 2001 just £4.1bn was spent on working-age adults mental health.

The report shows the shifting composition of mental health spending with investment in the three traditional priority areas – crisis resolution, early intervention and outreach – falling for the first time by almost £30m.

More than 6 million Britons are reported to suffer from depression and estimates put the economic cost of mental ill-health at more than £75bn a year.

Labour said the "the government's talk about the importance of mental health has come to nothing". A party spokesman said: "Ministers simply aren't putting their money where their mouth is and vulnerable people will pay the price when crucial services are hit.

"The prime minister has cut the NHS budget two years running and the impact of this is clear – millions less for mental health for the first time in a decade, thousands of nursing posts axed and treatments being rationed."

Charities argue that there is wide variation in mental health spending. The mental health charity Mind found that "considerable variation in service provision, for example with one in five people still waiting over a year to access to talking therapies whilst others are seen in under 28 days". Paul Farmer, chief executive of Mind, said: "This report shows a worrying trend ... during difficult economic times mental health is an easy target for cuts."

The DoH argues that the falls in spending reflect the fact it is treating people before they get seriously ill and need NHS-funded mental health provision. The report points out that investment in "psychological therapies increased significantly in real terms by 6% over the monies in 2010/11 and now forms 7% of direct services investment nationally".

A DoH spokesperson said: "Investment in psychological therapy has doubled over the last three years from £197m to £386m and continues to rise. This is allowing people to access psychological therapies earlier, which is driving up standards in care, reducing hospital admissions and delivering long-term savings. "Similarly, investment in memory assessment services is also rising, suggesting that the campaign to increase awareness of early signs of dementia is having an effect.

"However, we know we need to continue to improve the commissioning of mental health services. That's why we recently set out how organisations can make best use of the levers and powers available to them, to improve the mental health and wellbeing of local people in the most effective way."