Former veterans’ minister Johnny Mercer is potentially facing a prison sentence after his application to withhold the names of those who told him about alleged special forces murders was rejected by the Afghanistan Inquiry’s chairman.

Sir Charles Haddon-Cave’s ruling on Thursday quoted from the British Army’s values and standards by saying: “Integrity requires moral courage to do what is right, even when it may not be popular.”

Mr Mercer repeatedly refused to hand over names of “multiple officers” who told him about allegations of murder and a cover-up during his time as a backbench MP while giving evidence to the probe in February and has reiterated that he intends to “keep my word” to those who confided in him.

Mr Mercer claimed the ruling contained “multiple inaccuracies and assertions” and said he found it “extraordinary… that Mr Haddon-Cave considers it appropriate to repeatedly question my moral courage and integrity in public”.

Giving his reason for not disclosing the names during his evidence in February, Mr Mercer told counsel to the inquiry Oliver Glasgow KC: “The one thing you can hold on to is your integrity and I will be doing that with these individuals.”

In a previous order compelling the ex-Conservative MP for Plymouth to hand over the names, the inquiry chairman said the consequences of failing to comply without reasonable excuse would be “a criminal offence punishable with imprisonment and/or a fine”.

Mr Mercer then submitted an application to the probe on April 3, in which he argued he was either unable to comply with the order, or it was not reasonable for him to comply with it.

In his statement on Thursday following the ruling, which he posted on X, the former MP said: “I note the statement from the inquiry chair, containing as it does multiple factual inaccuracies and assertions.

“I find it extraordinary, given my unprecedented support to assist the Inquiry, including some names of those who have given me consent, that Mr Haddon-Cave considers it appropriate to repeatedly question my moral courage and integrity in public.

“These unjustified and unprovoked attempts by a High Court judge to assassinate my character, without any foundation whatsoever, crosses a line.

“From Northern Ireland to Iraq to Afghanistan, the courts have consistently mis-judged their handling of these issues, with appalling consequences for serving personnel and veterans, usually of junior rank.

“I could comment on the naivete of the inquiry team; or how – despite my repeated requests for them to have the basic courtesy to inform me privately before they release details of my fate to the press, I again found out my fate today from a journalist. But I would consider that improper.

“I have given my statement to the inquiry. Mr Haddon-Cave must now stop fixating on attempting to bully me, I have done nothing wrong. I gave my word to junior personnel who approached me. I intend to keep it.”

In his ruling on Thursday, Sir Charles said: “The Applicant submits that he is a protector of whistleblowers.

“He chose publicly, however, to disclose that friends told him about allegations of unlawful killings by (special forces) in Afghanistan.

“He has since refused to disclose the names to assist the Inquiry, even though: the Inquiry was set up for the very purpose of looking into these allegations; he says that his friends were merely witnesses; he could pass on their names to the inquiry privately and in strict confidence; he accepts that the inquiry protects the identities of confidential contacts; and they have protection from risk of prosecution for breach of the Official Secrets Act or failure to report misconduct.”

An inquiry spokeswoman said: “Mr Mercer is refusing to disclose information which may be important to a public inquiry which is seeking to establish the truth about grave allegations of multiple murder involving UK Special Forces.

“Mr Mercer accepts the inquiry has secure measures in place to protect the names and identities of his sources and that witnesses coming forward to the inquiry have protection from risk of prosecution for breaches of the Official Secrets Act or for failure to report misconduct.

“The chair has given Mr Mercer a further two weeks to comply with the Section 21 notice.

“The chair’s concluding observations were that ‘Integrity requires moral courage to do what is right, even when it may not be popular’.”

Mr Mercer now has until July 25 to hand over the names to the inquiry.