Eleven staff members from the Ministry of Defence have been sacked in the past two years for committing fraud on their expenses.
New figures show the MoD topped a list of six government departments that have launched investigations into expenses fraud, according to freedom of information requests.
The department investigated 378 breaches of expenses policy, the vast majority of which were errors, while fewer than five staff were prosecuted in relation to fraudulent claims.
The MoD said cases of fraud were rare and it had rigorous checks in place to catch those cheating the system.
Former defence minister, Kevan Jones said: "The high number of serious incidents of false claims, particularly within the Ministry of Defence, which is facing a budget shortfall, is concerning.
"This is an issue that requires urgent attention and ministers should be open about the scale and what they plan to do about it."
Figures previously obtained by Mr Jones through a written parliamentary question show 1,038 MoD personnel have been investigated on suspicion of fraud since 2010, with 151 resulting in convictions.
Over the same timeframe, the cost of fraud to the public was identified at around £1.7 million, with around £1.2 million having been recovered.
The MoD has more than 200,000 employees including military, reserves and civilian staff.
A spokesman said: "The vast majority of investigations found that there were errors made in submitting expenses, rather than fraud.
"Cases of fraud are rare but we run a proactive programme of work to detect potential fraud and error within our travel claims system.
"Rigorous checks, such as audits and random sampling, are in place to make sure that fraud is detected and deterred."