UK Defence Secretary Geoff Hoon also promised on Monday further reinforcements to help restore the country's crippled infrastructure.
The deployment, which will boost Britain's military presence in southern Iraq to 12,000, comes after leaked documents showed the government believed more troops were urgently needed to avoid a security collapse in Iraq.
"There is an immediate requirement for two battalions and some additional specialist personnel," Hoon said in a written statement to parliament.
The announcement followed a US plea for more non-American troops to counter violence and share the cost of occupying Iraq.
Washington wants another 15,000 soldiers from other nations to back its own 130,000-strong force.
The British reinforcements will stay until November and could be replaced "if the requirement remained".
Nearly 11,000 British troops have been supporting the US-led occupation since the overthrow of Saddam Hussein in April, but have come under regular attack.
Bush wants non-US soldiers to
bolster the occupation of Iraq
Hoon said there had been a "disturbing increase in terrorism against civilian targets", but Britain would not be deflected from the long term goal of bringing stability to Iraq.
Fifty British soldiers have died in the Iraq war and occupation, 17 of them in the last four months.
Meanwhile, the son of a top Iraqi Shia leader confirmed on Monday that his father, Ayat Allah Bashir al-Najafi, was the target of an assassination attempt.
A man arrested over the assassination bid on Saturday admitted "belonging to a network," Ali al-Najafi said.
The son identified the detained man as "Muhammad Saheb, 20, from the al-Tobji quarter in Baghdad".
"He confessed that he was on a mission to observe and find a way to assassinate (Ayat Allah Najafi) if the occasion permitted," al-Najafi said.
After interrogation, the man was handed over to US forces because he "confessed to killing two American soldiers".
Iraqi security forces
In a statement issued on Monday, the Najaf-based Hawza, Iraq's highest Shia religious authority, said the failed assassination attempt highlighted the need to boost Iraqi security forces.
The statement implicitly blamed Sunni Muslim radicals for the recent spate of attacks on Shia clerics and holy sites.
Iraq's highest Shia body blames
US for worsening security in Najaf
"The Hawza holds the (US) occupation forces and the (Iraqi interim) Governing Council responsible for preserving the security of the country and its men," the statement added.
A 400-strong protection force, whose members thwarted the attempt on al-Najafi's life, formally began work on Monday around one of Shia Islam's holiest shrines.
But members expressed regret that it had received US blessing too late to prevent a massive car bomb that killed 83 people on 29 August.
Arab League accord
And in a separate development, Arab foreign ministers in Cairo are close to reaching an accord on allowing an Iraqi delegate to attend their meeting as an observer.
But an Arab League official said on Monday this does not mean recognition of Iraq's US-backed Governing Council.
The accord under consideration provides observer status for the transitional Governing Council, and Iraq's interim foreign minister, Hoshyar Zebari.
This form of representation will continue "until an Iraqi government is elected by the Iraqi people to represent them," the official added.