The government's attempts to speed up the planning system have met with mixed results, the National Audit Office (NAO) has concluded.

In a report on Wednesday, the watchdog said that the number of major housing developments receiving a planning decision within the target of 13 weeks has nearly doubled over the last five years, reaching 67 per cent in 2007/08.

But Tim Burr, head of the NAO, said: "Whether the speed of development has increased is less clear."

The Department for Communities and Local Government (CLG) only records the number of applications that met the target, and not the number of weeks it took to reach these decisions.

It is therefore impossible to tell how long it takes, on average, for a major residential application to be determined, or how much this figure has reduced over time.

The NAO also reported that the target regime has "resulted in some cases in perverse consequences".

The report noted that developers had reported an incentive for local authorities to delay validating submitted applications to prevent the 13 week target period from starting.

There was also "a lack of incentive for authorities to tackle applications once they had missed the target".

And unresolved issues could be attached as conditions to permissions so that approval could be given within 13 weeks.

CLG has allocated approximately £110m to councils to boost their planning performance, but for much of the cash the NAO reported that "the extent to which it resulted in extra expenditure on planning is unclear, as authority finance officers tend over time to make allowances for such funding when setting planning departments' budgets".

There were further concerns about the introduction of a new standard application form, which was viewed as being too complex for smaller developments.