The Parliamentary Group for Muscular Dystrophy says it is concerned about funding for high-cost drugs after the special budget for treating rare diseases in England was merged into the overall NHS budget.
A UK-wide strategy for rare diseases will be published this year although Scotland has a fund in place for the so called "orphan drugs" until April 2014.
Millions of rare disease sufferers
Overall it is estimated that 3.5 million people in the UK suffer from a rare disease, and that 70,000 of those have some form of the different muscle-wasting diseases known as muscular dystrophy.
The MPs say they are concerned that funds previously ear-marked for drugs for rare diseases have now been merged into the budget for all specialised services in England.
Their call has been backed by Robert Meadowcroft, chief executive of the Muscular Dystrophy Campaign, who says they are gravely concerned at the lack of a dedicated fund or clear criteria for how new drugs might be assessed.
"We've got families setting great store by the treatments coming through. It would be heartbreaking and devastating if they're not available to children who need them."
Funds for 'orphan' conditions
Decisions on how to spend a £100m fund for rare diseases, sometimes called orphan conditions, were made on the advice of a special committee but that power was transferred to NHS England in April.
At the same time, the cost and clinical advisory body, the National Institute for Health and Care Excellence (NICE), took over responsibility for assessing these drugs as part of the wider reorganisation of the health service in England.
Agonising waits for families
The chairman of the all-party group, Dave Anderson MP, said: "We have seen that successfully developing an effective treatment is far from the end, with agonising waits for some families through licensing and funding issues."
NICE said it accepted it needed to develop a different approach for looking at rare conditions, adding: "Our process and methods for developing guidance for such drugs will ensure that patient and clinical experts are involved."
Negotiations are under way with the pharmaceutical industry on a new pricing regime which would include a new way of recognising the value of some new treatments.