The Daily Telegraph paid a mole £110,000 for the expenses files which shone a light into the murky world of MPs' claims and allowances, the newspaper's assistant editor revealed today.
Andrew Pierce defended the decision to pay for the leaked documents, describing it as "money well spent in the public interest".
He told BBC Radio 4's Today programme: "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; and I think the Telegraph took the decision not lightly, but we were given 24 hours to read that file - it blew our minds when we saw what was in that file."
His comments came as it was claimed that the mole, who has not been named, was angry at the Government's failure to invest in Britain's armed forces
The civilian employee broke ranks to reveal that politicians were lavishing millions of pounds of taxpayers' money on second homes and phantom mortgages.
The Daily Telegraph said the mole took 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.
In the book No Expenses Spared, to be published today, 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 Daily Telegraph 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."
A Ministry of Defence source cast doubt on the claim that serving soldiers were working at the Stationery 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.
The MoD also said its "top priority" was to provide servicemen and women fighting in Afghanistan with "the best equipment".
And Gordon Brown told Sky News he did not understand the motivation behind the leak.
He said: "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."
But Mr Pierce said the newspaper had "clear evidence" that soldiers were members of the security team hired as part of the redaction process, and that they were working to buy vital kit.
Some were working to earn "a little bit of extra money to pay for their kids' Christmas presents", Mr Pierce said, adding: "But others were there to pay for essential body armour, gloves, boots, to serve in Afghanistan.
"We have got clear evidence of that.
"You know they don't get paid very much, there has been lots of evidence before where soldiers have bought their own materiel, and we have seen in inquests that sometimes soldiers have died in Afghanistan or Iraq because they didn't have the right equipment."
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 items 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 the Prime Minister'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 are 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."
An MoD spokesman insisted that service personnel receive all the equipment they need.
"Every soldier who deploys to Afghanistan receives Osprey body armour and a Mark 6a helmet," he said.
"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."Reuse content