Andrew Pierce: "These were soldiers, who get paid a pittance, absolutely dismayed"
The mole who leaked details of MPs' expenses says he was partly motivated by anger at inadequate equipment for UK troops, the Daily Telegraph reported.
It says staff sorting through MPs' receipts were guarded by servicemen on leave moonlighting to earn extra money.
The mole said their stories "helped tip the balance" in the decision to leak details - the Telegraph has confirmed it paid £110,000 for the information.
The MoD says its top priority is to get the "best equipment" for troops.
The claims are made in a book being published by the Daily Telegraph on Friday about the expenses scandal.
The newspaper leaked details of what MPs had been claiming on their second homes expenses in May - the Commons had been ordered to compile receipts and claims made over four years by the High Court, following a long-running Freedom of Information battle.
After the story broke, party leaders agreed interim reforms to the system, many MPs agreed to repay money and some have since announced they will not be seeking re-election.
I have been determined to make sure that the troops that are serving our country are properly paid, that we give them the best equipment, that we help them in every way possible
Andrew Pierce, the Telegraph's assistant editor, told the BBC the payment to the source of the leak was an "insurance policy" for the mole, as that person would lose their job if the government discovered their identity. He also said it was "cheap at the price" as MPs had already repaid hundreds of thousands of pounds.
He said: "We paid £110,000 to the source. And let me just say, so far the taxpayer has been reimbursed by MPs £500,000, and there will be more; we have got a much better Commons as a result of it."
Mr Pierce added that servicemen had overheard staff working on MPs' receipts: "As civil servants were redacting, or censoring, or covering up, or Tippexing out up the difficult details, they were exclaiming out loud to each other... 'Oh my god, can you see what they've claimed for?'"
Sources at the Ministry of Defence (MoD) have cast doubt on whether serving soldiers would be allowed, or have the time, to moonlight as security guards. But Mr Pierce said the newspaper had "clear evidence" some were doing extra work to buy equipment.
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 mole claimed the contrast between conditions facing soldiers and the MPs' claims "helped tip the balance in the decision over whether I should or should not leak the expenses data".
Every soldier who deploys to Afghanistan receives Osprey body armour and a Mark 6a helmet
Asked on Sky News if he understood the motivation for the expenses leak, Prime Minister Gordon Brown said: "I don't think so."
He added: "MPs have got to live in two places at once - that is a big problem.
"As far as the troops in Afghanistan are concerned, right throughout the period I have been chancellor and then prime minister, I have been determined to make sure that the troops that are serving our country are properly paid, that we make proper allowance for them, that we give them the best equipment, that we help them in every way possible."
The MoD said: "Since 2006, we have delivered equipment valued at more than £10bn to the armed forces. "Every soldier who deploys to Afghanistan receives Osprey body armour and a Mark 6a helmet.
"They also receive a black bag containing all their operational requirements. Valued at £3,500, it contains everything a soldier will need from boots and socks to camel backs."
The Telegraph says about 20 of Her Majesty's Stationery Office and parliamentary staff worked for months in great secrecy in a south London office copying and processing the MPs' receipts.
The BBC's political correspondent Ben Wright said although there was originally a hunt to find the mole when the scandal broke, it was unsuccessful and has since been called off.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.