The mole who leaked details of MPs' expenses claims was angry at the Government's failure to invest in Britain's armed forces, it emerged.
The civilian employee broke ranks to reveal how politicians were lavishing millions of pounds of taxpayers' money on second homes and phantom mortgages.
The Daily Telegraph said the mole, who has not been named, took the action after being involved in the processing of MPs' expenses files - at the same time as serving soldiers were "moonlighting" as security guards at his office to earn extra money for body armour and other equipment.
It said the soldiers' "fury" at the way MPs were "lavishing taxpayers' money on their second homes" led to the decision to leak the data, via a middleman, to the newspaper.
The account appears in the book No Expenses Spared, which will be published today.
The newspaper said: "The man behind the leak - who is a civilian - has broken cover to tell his story for the first time, in the hope that it will shame the Government into finally supplying the right equipment for soldiers risking their lives in Afghanistan."
The MPs' expenses revelations, published in the Daily Telegraph, led to widespread recriminations.
It exposed the "flipping" of second homes, expense claims on paid up mortgages and tax avoidance. Moat cleaning and a duck island were among the things MPs claimed public money for.
The newspaper said employees at The Stationery Office "became so agitated that they had to be told by managers to calm down" after reading the claims.
They had been involved in processing the files for censorship before their intended publication by parliament.
The newspaper said one file that "particularly enraged" the employees was Prime Minister Gordon Brown's claim for a Sky TV sports package, which cost £36 per month.
It said the mole was angered by politicians who, five months after the expenses scandal broke, "still don't get it", and were still more concerned about their own financial affairs than the plight of troops.
The mole said: "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.
"Hearing from the serving soldiers, about how they were having to work there to earn enough money to buy themselves decent equipment, while the MPs could find public money to buy themselves all sorts of extravagances, only added to the feeling that the public should know what was going on.
"That helped tip the balance in the decision over whether I should or should not leak the expenses data."
Asked on Sky News if he understood the motivation for the expenses leak, Mr 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."
A Ministry of Defence spokesman said its "top priority" was to provide servicemen and women fighting in Afghanistan with "the best equipment".
He said: "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."
A MoD source cast doubt on the mole's claim that serving soldiers were working at The Stationary Office's headquarters where the job of redacting more than 1.5 million MPs' expenses receipts was carried out.
The source said there are strict rules governing second jobs for troops - they must be declared and soldiers must have the permission of their commanding officers.Reuse content