The Ministry of Defence (MoD) has decided to extend its cyber security agreement with BT.
BT said the contract's value could not be revealed for security reasons but said that work had already begun on the seven-year arrangement with the MoD.
As part of the extension, BT will deliver and integrate new capabilities that are designed to deter, react to and recover from any attack or attempted exploitation.
In 2010, the MoD detected and blocked more than 1,000 "potentially serious" attempts to infiltrate or disrupt its computer systems, more than double the amount found in 2009.
Air commodore Tim Bishop, head of service operations at the MoD, said the growing threat from cyber attacks means that there needs to be constant improvements in the MoD's defence capabilities.
"Our forces depend on computer networks, both in the UK and in operations around the world. Over the last five years we've seen a significant rise in the number of threats to the UK's national security through cyberspace, so it is crucial that we look to constantly improve our defence mechanisms to meet those threats," he said.
"BT has already delivered a world-leading solution, so it makes sense to develop our relationship further," he continued.
In April, deputy head of service operations at the MoD, captain Simon Wise, said that there was more risk than ever of data being compromised, while last year defence secretary Liam Fox stated that the MoD faced a "continuous battle" in cyberspace.
BT said that the extension will build on its working relationship with the government to help deliver the UK National Cyber Security Strategy, which was announced in 2009.