MPs and unions say inquiry is heavy-handed and a waste of money after photographs show stacks of passport applications

The Passport Office has ordered an inquiry into photographs leaked to the Guardian which showed hundreds of files, full of passport applications, stacked in a room usually used for meetings.

The move, which could lead to disciplinary action, has been described as heavy-handed, a waste of money and against the public interest by MPs and union officials.

Photographs, taken in a conference room in the Passport Office centre in Liverpool, were published as the agency's chief executive, Paul Pugh, was denying there was a backlog of cases.

A day after their publication, David Cameron admitted for the first time that there was a problem in the Passport Office. Theresa May, the home secretary, on Thursday launched a series of measures to ease the backlog, which the government puts at 30,000 "straightforward" applications.

Andy Bannon, the security liaison officer, sent a letter to staff on Wednesday saying: "I am sure everyone is now aware of the photographs of the temporary storage facility on the second floor that were leaked to the press yesterday.

"This is being viewed as a serious security breach and I have notified the internal investigations team accordingly. They will be making a full investigation and if there is clear evidence linking a member of HMPO, Steria or our contracted staff to the breach, formal misconduct procedures will be considered," he wrote.

"Whatever the motives behind the leak, there does not appear to be any recognition of the impact this has had on colleagues working exceptionally hard to manage the unprecedented levels of demand currently being experienced."

He added that any further security breaches could result in a blanket ban on mobile phones on the premises.

John Mann, Labour MP for Bassetlaw, said the person who took the photographs should be congratulated by the Passport Office because their actions had revealed the extent of problems that were being denied.

"Over the last few weeks I have had to intervene to get a 90-year-old war veteran a passport so he could get to the D-day celebrations after the Passport Office failed to process his application. Children in my constituency have missed their school trips because of this mess.

"The hero who took these photos has offered my constituents and millions of others hope that this shambles might not continue. Instead of wasting resources chasing a genuine whistleblower, the Passport Office should spend their time and money getting people their passports," he said.

Mark Serwotka, the head of the PCS union whose members represent staff in Britain's seven Passport Office centres, said the move was heavy-handed and against the public interest.

"This is a ludicrously over-the-top response from the Passport Office and will do nothing to restore the goodwill that has been lost in recent days and weeks, as officials and ministers have sought to cover up the extent of the problems caused by a chronic shortage of staff.

"Arguably if it wasn't for those photographs, we wouldn't have had the debate in the Commons today and there was clearly an overwhelming public interest in publishing them," he said.

The photographs show hundreds of boxes of orange and white files stacked on tables. A source close to the Liverpool office said there were 15 to 20 applications in each box, and that the orange boxes were overseas applications while the white boxes showed those from the UK.

The room is usually used for meetings and induction sessions and is usually partitioned into two, the source said. It was not used to store application files during the peak period last year and had not been used before for this purpose.

Two other meeting rooms on the first floor of the building and another second-floor meeting room were also being used to store applications waiting to be processed, said the source.

Insiders said this was the first time the room had been used to store files of applications, forcing them to hold meetings in a stationery cupboard.

A Home Office spokesman said the department would not comment on a leaked document.