A British soldier is seen on patrol in Basra, 550 kilometres southeast of Baghdad, in July 2008. Britain formally ended six years of combat operations in Iraq on April 30, 2009. (Nabil al-Jurani/Associated Press)

United States officials assumed Britain would take part in a war in Iraq even without United Nations approval, the U.K. probe into the invasion heard on Thursday.

Adm. Michael Boyce, Britain's defence chief in the lead-up to the March 2003 invasion, said U.K. officials repeatedly suggested to their American counterparts that they needed to go through the UN and the British Parliament before committing troops.

But U.S. Defence Secretary Donald Rumsfeld and other American officials assumed Britain would be there, Boyce said Thursday.

"There was a huge reluctance by the U.S. throughout, from July 2002 through to March 17, 2003, to believe that we were not going to commit our forces unless we had been fully through the UN process and through Parliament as well," he said.

He said the American reaction was "'You have got to say that, but actually come the day, you will be there'. That was the attitude."

Probe looks at lead-up to invasion

The inquiry, which began last week, is looking into whether former U.K. prime minister Tony Blair pledged support for the invasion of Iraq before Parliament approved military involvement in 2003.

The panel hearing testimony at the inquiry won't lay blame or establish criminal or civil liability but will give reprimands if warranted and make recommendations. Blair is scheduled to testify in 2010.

Much of the attention of the probe so far has focused on the April 2002 meeting between Blair and U.S. President George W. Bush at Bush's ranch in Crawford, Texas. Several diplomats have told the hearing they believed the two leaders made an agreement to take military action at that meeting.

Boyce said Britain's military chiefs began planning for potential involvement in an American-led invasion after the Crawford meeting, and a year before Parliament authorized involvement.

"We started ramping up our thinking on the whole subject, of what we could provide if we were asked," he said.

High-level planning only: Boyce

Boyce said the planning remained high-level only, and said it was frustrating for military officials, who could plan but not work out the logistics as Britain attempted diplomatic efforts with the UN.

The UN passed a resolution in November 2002 that paved the way for inspectors to return to Iraq to verify that it was not developing nuclear weapons. However, Britain and the U.S. were unable to get a second resolution passed in March 2003 to authorize the use of military force in Iraq if then leader Saddam Hussein refused to let inspectors into the country. The invasion began days later.

The inquiry also heard from Kevin Tebbit, the permanent secretary at the Ministry of Defence until 2005, who said military chiefs hadn't pushed for British involvement in the war.

"At no stage, frankly, did I feel that there was an effort by the military establishment to drive the agenda," he said. "Whether that was the case in the United States, I cannot say."

The British military formally ended six years of combat operations in Iraq on April 30, 2009, handing over control of its main base in Basra to a U.S. brigade.

A total of 179 British soldiers lost their lives during the U.K. involvement in Iraq.

With files from The Associated Press