Saddam Hussein's regime was not the British government's main concern in 2001, a senior Foreign Office official said.
Ehrman says in remarks before the country's Iraq Inquiry that Britain was focused on threats from Iran, Libya, and North Korea.
Ehrman -- the director of international security from 2000 to 2002 -- spoke Wednesday in the inquiry billed as the most sweeping look at the conflict by any nation involved.
Tim Dowse, the Foreign Office's head of counter-proliferation, has also testified that Iraq wasn't ''top of the list.''
The inquiry could expose alleged deception in the buildup to fighting but won't establish criminal or civil liability, The New York Times reports.
Meanwhile, British Defence Secretary Bob Ainsworth announced a public inquiry Wednesday into allegations that Iraqis were killed and abused after a firefight more than five years ago in the southern part of the country.
The allegations center around the aftermath of a fight in May 2004 at the so-called Danny Boy checkpoint in Maysan Province.
Former detainees and the family of a slain Iraqi contend at least 20 people were killed and others were abused at Camp Abu Naji after a fight between British soldiers and Iraqi insurgents.
That claim has been denied by the UK Ministry of Defence -- which says the 20 people died in battle and people detained were not mistreated, CNN reports.
William Ehrman, the then director of international security at the foreign office, said the US had "put more weight on some of the links" than Britain did.
"But our view was there was no evidence to suggest serious collaboration of any sort between al-Qaeda and Saddam Hussein's regime," he said.
Dowse said the assessment was shared by colleagues in US intelligence, but suggested this may not have been the case among some in the Bush administration.
The second day of the inquiry, which is expected to last for up to a year, also addressed claims by the UK government that Baghdad was developing weapons of mass destruction in the lead up to the conflict in 2003.
Dowse said he had not been surprised by the 45-minute claim "because it didn't seem out of line [with assessments at the time]", Aljazeera.net reports.
US President Joe Biden and Iraqi Prime Minister Mustafa Al Qadimi signed an agreement on July 26 to formally end the USA's military presence in the country by the end of the year