Ordinary taxpayers and the government finances could face "havoc" due to HM Revenue & Customs' complacent approach to IT systems, MPs have warned.
The Public Accounts Committee (PAC) said the taxman faced an "enormous challenge" in replacing the Aspire contract for maintaining its hardware and software by 2017.
The department does not currently have the skills to negotiate good value new IT deals - and also lacks a contingency plan in case there are serious issues.
In its report, the PAC said the Aspire contract with Capgemini was the Goverment's largest technology deal and had been expensive - costing £7.9 billion over the past decade.
However, it had provided "stable systems to support the collection of taxes", amounting to over £500 billion in 2013-14.
"HMRC faces an enormous challenge in moving to a new contracting model by 2017 and appears overly complacent given the scale of the transformation required," the report warned.
"Although HMRC decided three years ago to move in principle to a new contracting model it still does not have a detailed business case for the change although this was originally expected before July 2014."
The MPs highlighted difficulties in complying with new Cabinet Office rules for using a wider range of smaller-scale IT suppliers.
"The end of the Aspire contract and the move to replace it with many more contracts and suppliers puts both the service HMRC provides to customers and tax collection at risk," the said.
"A failure of HMRC's ICT could put at risk the timely and accurate collection of tax, potentially resulting in reduced revenue."
The report pointed out that problems with a previous move to centralise HMRC's PAYE and National Insurance databases had resulted in nearly £1 billion of tax being foregone.
"HMRC is aware of the risks involved in replacing the Aspire contract but it has not quantified them," the MPs said.
Senior committee member Richard Bacon said: "HMRC's record in managing IT contractors gives us little confidence that HMRC can successfully achieve this transition or that it can manage the proposed model effectively to maximise value for money.
"The provision of all government services depends on the continued stability of tax collection, which yielded more than £500 billion in the last completed financial year and, on the same trend, would be expected to yield around £5 trillion over the next ten years.
"There are substantial risks to tax collection if the transition fails, which would create havoc with the public finances."
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