The Government's drive to achieve "parity of esteem" between mental and physical health services in England will be "very difficult to achieve" given the financial constraints on the health service, MPs have said.
The Public Accounts Committee said the aim is a "laudable ambition" but pressure on the NHS budget has made them "sceptical about whether this is affordable".
A new report by the influential group of MPs also states the way mental health services are designed is "complex, variable and difficult to navigate", which can make it difficult for people to access the care that they need.
The MPs say that achieving parity of esteem will depend on the health system having the right staff in place but "there is not yet a clear plan to develop the workforce needed".
"The Department of Health and NHS England have a laudable ambition to improve mental health services but, given the current pressures on the NHS budget, we are sceptical about whether this is affordable, or achievable without compromising other services," the report states.
It adds that "pressure on the NHS budget will make it very difficult to achieve".
Meg Hillier, chairwoman of the committee, said: "The Government has committed to making much-needed improvements to mental health services but we are concerned it does not yet have sound foundations to build on.
"As a priority the Department of Health and NHS England must achieve a better understanding of the current landscape and the likely costs of achieving its goals.
"If these goals prove beyond the scope of the funds provided then it is vital a plan is in place to make best use of the money available."
A Department of Health spokesman said: "As the Public Accounts Committee has recognised, we have made important progress in the way we think about and treat mental illness in this country, introducing the first mental health waiting times in the history of the NHS.
"We are determined to accelerate progress further working across Government, and are backing the NHS plan to revolutionise mental health care with an additional £1 billion invested every year by 2020 - helping one million more people with mental illness access high quality care."