PlayGordon Brown has warned that ministers are prepared to take action against contractors who lose sensitive Government data.
Officials were forced temporarily to shut down the key Gateway website - used by millions of people to access Government services - after a memory stick containing confidential passwords was found in a pub car park.
Contractors Atos Origin, which manages the system for the Government, admitted that the device had been removed from the company's premises by one of their employees in "direct breach" of its operating procedures.
The Prime Minister, who is on a tour of Gulf states, said that it was "unacceptable behaviour".
"I think the important thing is to prevent these kind of things happening in future.
"There will have to be changes in the way these contracts are issued to companies that make mistakes like this."
The Government's embarrassment was compounded by the disclosure that Work and Pensions Secretary James Purnell - whose department is responsible for Gateway - had to apologise after leaving confidential documents on a train.
Mr Purnell mislaid the letters, relating to the case of a constituent of Labour MP Sir Gerald Kaufman, while he was working on his ministerial "red box" during a journey from Macclesfield to London's Euston Station on October 4.
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