The new IT system is used by troops in the field
Delays in installing a new £7bn computer network has left the Ministry of Defence (MoD) reliant on outdated systems, MPs have warned.
The Defence Information Infrastructure (DII) was supposed to replace hundreds of existing systems with a single one for warships and troops.
But the Public Accounts Committee said that the project was over budget and 18 months behind schedule.
Chairman Edward Leigh said the delay could make troops "more vulnerable".
The DII aims to provide a system of 150,000 terminals across 2,000 military sites, including Royal Navy warships and with troops deployed on operations.
But the committee said that the 10-year project, started in 2005, only had funding for 140,000 terminals.
It blamed "totally inadequate research" which failed to take into account the poor state of many military buildings, where problems such as asbestos had to be dealt with before the equipment could be installed.
Of the 62,800 terminals which should have been installed by the end of July 2007, only 45,600 were in place by the end of September 2008.
Although the rate of installation had since been increased, the committee said work was still behind schedule.
Mr Leigh told the BBC: "If you are supposed to have delivered over 60,000 terminals and you have only delivered 40,000 then presumably some people are more vulnerable than they should be."
The committee also questioned the government's choice of the Atlas consortium, which is headed by the US firm EDS, to deliver the project.
EDS has been responsible for other troubled Government IT projects including the notorious Child Support Agency system.
Mr Leigh described EDS as "a company whose track record of delivering Government IT projects has not been exemplary".
He said that the consortium had "underestimated" the complexity of the software it was creating.
He said that the MoD must now take steps to protect against failure of the old systems while DII is completed, paid for out of Atlas's management fees.
"Given the scale of delay, the department must head off the risk that existing IT systems, upon which MoD staff and military personnel rely, will fail," he said.