telegraph.co.uk | Sources in Washington said the inquiry sent out emails "about three weeks ago" to senior officials in Mr Bush's government including, it is believed, the former president himself.
Other requests are understood to have been made to Dick Cheney, Mr Bush's vice-president, Condoleezza Rice, the former secretary of state, Donald Rumsfeld, the former US defence secretary, and Stephen Hadley, an ex-national security adviser – as well as to their deputies and senior assistants.
Members of Sir John Chilcot's panel are believed to be willing to travel to the US to take evidence – almost certainly in private – on the administration's policies between the 2003 invasion of Iraq and 2009.
While the most senior figures are reluctant to give evidence, Washington sources claim about 10 former officials, most involved in the post-invasion period, have agreed to do so.
The surprise development adds to the chances of Sir John's inquiry producing a "smoking gun" on the key questions of whether Britain and the US adequately prepared for the conflict and whether it was justified under international law.
Interviews with US officials – even held in secret – could play a major part in Sir John's final report, expected by the end of this year.
Although it has no legal power to compel witnesses to appear before it, the Chilcot Inquiry has succeeded in obtaining testimony from virtually every single British politician, official and senior military figure who played a key role in the war.
Both Tony Blair and Gordon Brown have been subjected to six-hour televised grillings.
The Prime Minister may even have to return to the inquiry to "clarify" his previous evidence after admitting providing wrong information in his earlier appearance.
Last week, Mr Brown told MPs he had been wrong to tell the Iraq Inquiry that defence spending under Labour was 'rising in real terms every year'.
A research paper produced by the House of Commons library shows defence spending fell in real terms in four years when Mr Brown was Chancellor, including two when Britain was at war in Iraq and Afghanistan.
Mr Brown has come under fire from a battery of former senior military figures who claim defence was underfunded during his years at the Treasury.
Other notable witnesses who have already given evidence to Chilcot include Alastair Campbell, the former communications director at 10 Downing Street, who told the inquiry that Mr Brown was a key member of Mr Blair's "private circle".
However, others said Mr Brown was a rallying point for dissent about the war. Clare Short, the former international development secretary, described frequently sharing her concerns with a "very unhappy and marginalised" Mr Brown.
Mr Blair mounted a vigorous defence of the invasion and insisted he had no regrets over removing Saddam Hussein. The former prime minister denied he took Britain to war on the basis of a 'lie' over the dictator's supposed weapons of mass destruction.