Image caption The inquiry is investigating the background to the invasion of Iraq in 2003

The Foreign Office (FO) has declined to comment on claims by a former diplomat that it blocked key parts of his testimony to the Iraq inquiry.

Carne Ross, the UK's Iraq expert at the UN from 1997-2002, said the FO withheld documents he requested, and warned him not to refer to a key memo.

He told the Observer newspaper that he was subjected to a form of "subtle intimidation".

The FO said "we are not going to comment on what witnesses have said".

'Not plausible'

Mr Ross, who appeared before the Iraq inquiry earlier this month, alleged that "deep state" elements were preventing the inquiry from finding out the true reason for the invasion of Iraq in 2003.

The government is confident the inquiry will play a vital role in learning lessons from Britain's engagement in Iraq Spokesman, Foreign Office

He said he had planned "to detail how the UK government failed to consider, let alone implement, available alternatives to military action".

But Mr Ross alleged he was prevented from doing this because the FO withheld documents; including one he claimed related to a visit to Syria by former prime minister Tony Blair, and another on the failure to deal with a pipeline he said enabled Iraq to illegally export oil through Syria.

He said he was told such documents could not be found - an explanation he told the Observer was "simply not plausible".

Mr Ross further alleged that an official requested he did not tell the inquiry about an internal FO memo he claimed criticised a letter prepared by the government for the Parliamentary Labour Party for "dramatically - and inaccurately" exaggerating Iraq's nuclear threat.

He claimed that the letter, which was instead sent to the cabinet, was "pure overstated propaganda".

Mr Ross added that while there was "some truth" to the argument that the military alternative to deal with the Iraqi threat was more or less unavoidable, "it was not what the Foreign Office, or the government as a whole, believed at the time".

'Independent'

A FO spokesman said: "We are not going to comment on what witnesses have said, why the inquiry has called them, or what their lines of investigation should be.

"These are matters for the independent inquiry to determine."

The spokesman added: "The government is confident the inquiry will play a vital role in learning lessons from Britain's engagement in Iraq.

"The inquiry will be comprehensive and independent. It is not a trial or an impeachment, but an effort to learn for the future."

Mr Ross also claimed that the Iraq inquiry panel, chaired by Sir John Chilcot, was "neither equipped, nor apparently inclined" to challenge witnesses "on the contradictions of their testimonies".

However, he said this "may not be the panel's fault", because he alleged members did not know of the existence of key documents.

Mr Ross claimed: "In these circumstances, it is very worrying that the government machine is still trying to withhold key documents and silence those of us with detailed knowledge of the policy history - and documents."

A spokesman for the inquiry said Mr Ross had not notified it of any documentation he wished to have declassified.

He added that the inquiry was happy it was being given all the necessary documents.

Mr Ross resigned from the Foreign Office in 2004, and is now executive director of Independent Diplomat, an organisation which gives independent diplomatic advice.