The expenses scandal began because the mole who caused the leak was angry about inadequate equipment for the armed forces, the Daily Telegraph said.
The newspaper says staff who worked in secrecy on MPs' receipts were guarded by servicemen on leave between tours.
They became angry after seeing MPs' lavish claims when soldiers had poor equipment, prompting a civil servant to leak the story, the paper says.
But Gordon Brown said he did not understand the motivation for the leak.
The unidentified mole is quoted as saying the soldiers' accounts of their poor equipment prompted him to leak the information.
According to the Telegraph, he claims that many of the service personnel were moonlighting so they could buy better kit.
Tipping point
The paper quotes the mole as saying: "It's not easy to watch footage on the television news of a coffin draped in a Union Jack and then come in to work the next day and see on your computer screen what MPs are taking for themselves."
The Ministry of Defence (MoD) said their "top priority is to provide the best equipment for our people in Afghanistan". read more
