The Home Office has refused to fully justify the £1.8bn price tag attached to its contentious draft Communications Data Bill, which if passed will massively increase online surveillance of UK citizens.

In response to a Freedom of Information request from Zoe O'Connell, who regularly blogs about the bill, the Home Office argued that it needed plenty of wiggle room with its figures: for example, the cost per megabyte to record Brits' online activities will decrease and internet traffic is soaring.

But the government department declined to produce documents used to calculate the estimated £1.8bn to build and run Home Secretary Theresa May's proposed web snooping system. Whitehall officials said showing their workings will "pose an unacceptable risk to the ability of the UK to safeguard national security" and "disclosure of this could be used to avoid detection".

However, the Home Office did reveal some more details about the business case used to "maintain capability to access communications data before and after the introduction of new legislation". More interestingly, it also made it clear that the estimated cost, which will be spread over 10 years, is highly likely to be revised:

The programme business case is currently undergoing a formal regular review which will produce revised costs and benefits. The revised business case will inform a new Impact Assessment to be published before the Communications Data Bill is introduced formally in the House. Parliament will then have the opportunity to consider again the latest costs and benefits of the programme enabled by the Bill. The technology delivery aspects of the programme have recently been reviewed by the Major Projects Authority in the Cabinet Office who have endorsed the Programme’s approach. The Home Office has provided for the programme business case in its spending plans until March 2015, after which it will be subject to further scrutiny as part of the next spending review.

Many have questioned how the Home Office came up with its figures. Officials said the price tag represents "a highest cost scenario". Labour's similar shelved Interception Modernisation Programme (IMP) was estimated to cost £2bn over the same period of a decade.

MPs and peers scrutinising the bill have been told Blighty's spy chiefs, who demanded new systems to snoop on internet communications in the UK, are out of touch with how quickly the tech needs to be updated.

The Home Office said in its FOI response that it was addressing this concern by saying that its planned system had "an incremental approach to developing capabilities, which takes account of changes in technology, the communications market and the investigative capabilities of law enforcement and intelligence agencies".

It also admitted on the £1.8bn price tag that it was "difficult to estimate costs with precision over the long term". ®