Basic accounting errors resulted in the Ministry of Defence (MOD) wasting millions of pounds when it decided to change the types of planes used on planned new aircraft carriers in a failed bid to reduce costs and increase performance.

The Public Accounts Committee (PAC) made the criticism in its scrutiny of MOD plans which were first announced in 2007 to acquire two aircraft carriers, available from 2016 and 2018, at a cost of £3.65bn. MPs heard the cost had now risen to some £5.5bn and the carriers would not be ready before 2020 at the earliest.

In October 2010 the MOD said it wanted to change the type of aircraft to be flown from the carriers, claiming the move would save money and improve capability. Eighteen months later, in May 2012, the department had to admit its calculations were wrong and revert to the original plans, having spent an additional £74m on the switch.

PAC chair Margaret Hodge said the MOD's decision was based on 'deeply flawed information, generated under time pressure and in secret'. This included what Hodge described as 'incredibly basic errors such as forgetting to include the costs of VAT and inflation'.

Hodge said: 'The committee is still not convinced that the MOD has this programme under control. It remains subject to huge technical and commercial risks, with the potential for further uncontrolled growth in costs.'

The PAC also criticised the MOD for its failure to negotiate properly with industry over the carriers, saying the current contract was not fit for purpose as it failed to provide industry with any real incentive to control costs.