Tony Blair is trying to rebuff his critics over the Iraq war with a speech on the threat of global terrorism.
Tony Blair says international terrorism is a threat to Britain
He is combining arguments for the war in Iraq and preventing the spread of weapons of mass destruction in Friday's speech in his Sedgefield constituency.
According to Downing Street, the prime minister feels too many people underestimate the terrorism threat.
Mr Blair is expected to say the public are in "mortal danger" of mistaking the nature of that threat.
Foreign Secretary Jack Straw, visiting Pakistan, repeated this belief that international terrorism represented a new scale of threat.
"We must not judge the challenges of the future against the standards of the past," Mr Straw told BBC Radio 4's Today programme.
"The profound difference is that in the past threats came from conventional sovereign states, and however terrifying, we knew how to deal with them. These new threats come from unconventional forces, very highly organised international terrorism."
Although there had been recent successes in limiting the proliferation of nuclear weapons, Mr Straw warned of the threat of terrorist networks such as al-Qaeda obtaining such weapons.
The prime minister has been seeking in recent weeks to re-focus on domestic issues.
But Iraq has remained at the top of the political agenda following Clare Short's claims British intelligence spied on UN Secretary General Kofi Annan, continuing concerns about the legality of the war and also the devastating bomb attacks in Karbala and Baghdad.
Iraq was the main subject discussed during talks in Rome on Thursday between Mr Blair and his Italian counterpart, Silvio Berlusconi.
Mr Blair said the situation in Iraq was of "fundamental importance" to the world's future security.
"Even if they disagreed with the war, people should understand what's happening in Iraq today," he said.
"We are trying to work with the Iraqi people to put the country on its feet.
"The struggle that's happening is a struggle that literally will be of fundamental importance to the future of the Middle East and to the future of the world."
Downing Street officials insist the prime minister is not being forced to make Friday's speech to defend his actions in Iraq.
But they say Mr Blair is convinced he needs to address the issue of international terrorism in detail for the good of the country.
The decision to go to war against Iraq faced a further challenge from former chief United Nations weapons inspector, Hans Blix, who claims that the war was illegal.
"I don't buy the argument the war was legalised by the Iraqi violation of earlier [United Nations] resolutions," he said.
But Mr Blix dismissed calls that either President Bush or Mr Blair should resign or face a tribunal over the war, saying that they were already paying a political price for their decision.