International terrorism and cyber attacks pose the biggest threats to British security, a new government strategy said Monday, just before deep cuts to the defence budget are unveiled.

Prime Minister David Cameron's coalition identified these as "tier one" threats in a new national security strategy alongside natural hazards such as flu pandemics or floods, and foreign military crises that may involve Britain.

The strategy was unveiled before Cameron announces details of a defence review Tuesday, which is set to outline cuts of around eight percent in the Ministry of Defence (MoD) budget.

Those cuts are part of Wednesday's comprehensive spending review which could see government-wide savings of up to 25 percent as the coalition, which took office in May, battles to pay off Britain's huge deficit.

Late Monday, Cameron called US President Barack Obama to reassure him that Britain would remain "a robust ally of the United States" and would continue to meets its commitments to military alliance NATO despite the defence cuts.

In a foreword to the security strategy, Cameron warned that Britain was entering an "age of uncertainty" where threats to its national interests were constantly changing.

"All of this calls for a radical transformation in the way we think about national security and organise ourselves to protect it," he wrote.

He added that the previous Labour government, which took Britain into wars in Afghanistan and Iraq, had left "a defence and security structure that is woefully unsuitable for the world we live in today".

The new strategy arranges the risks to Britain's security into three tiers which are likely to reflect how budget resources are allocated, although officials insist all the threats named were important and would be addressed.

The top tier includes international terrorism, which Cameron says is the "most pressing threat we face today".

This comes principally from Al-Qaeda, which the strategy says has been weakened in Afghanistan and Pakistan thanks to international military action but can exert its influence through affiliates in Somalia, Yemen and Iraq.

The document also warns of the threat of paramilitary groups in Northern Ireland, saying that "the activities of residual terrorist groups (opposed to the peace process) have increased in the last 18 months".

Cyber attacks are also considered a top tier threat, particularly given how dependent British businesses and services are on the Internet and with the 2012 Olympic Games in London likely to be a particular target.

"Cyberspace is already woven into the fabric of our society. It is integral to our economy and our security," the strategy says.

Last week, the head of Britain's electronic spying agency warned the country faced a "real and credible" threat of a cyber attack from hostile states or criminals.

In rare public comments in London, Iain Lobban -- director of the Government Communications Headquarters (GCHQ) -- said infrastructure such as power grids and emergency services was at risk.

Elsewhere in tier one are natural incidents, such as major flooding or the recent flu epidemic, and foreign military and humanitarian crises that could force Britain to intervene to protect its strategic interests.

On tier two, signalling a lower level of importance, is the risk of a chemical, biological or nuclear attack in Britain by a foreign state or proxy.

This comes above the risk of a large-scale conventional military attack, which was included in tier three alongside threats to energy security, the disruption of food supplies or a nuclear accident on British soil.

In his phone call with Obama late Monday, Cameron pledged that Britain "would remain a first rate military power and a robust ally of the United States," said Downing Street in a statement.

Cameron told Obama that Britain was "committed to meeting our responsibilities in NATO and would continue to work closely with the US on the full range of current security priorities," said a spokesman from the premier's office.