Former Home Secretary Jack Straw has criticised the Coalition Government's decision to send two million letters to Scottish households during the independence referendum.
The Labour politician told MPs the mail out used public money to send a "party political flier" to every house in Scotland.
The move "breached" strict rules around civil service impartiality, he added.
Mr Straw also suggested that the top official in the Treasury Sir Nicholas Macpherson had not abided by rules on neutrality.
His comments came as he gave evidence to MPs on the Commons Public Administration and Constitutional Affairs Committee who are looking into the rules on referendums ahead of 2017's European Union vote.
The SNP accused ministers of using taxpayers money to secretly fund the 'No' to independence campaign.
Mr Straw said: "That letter without any question breached purdah rules and it probably breached normal rules of public spending as well.
"It was a party political flier sent to two million households and it was wrong for the government to do it."
Labour MP Paul Flynn told him that a senior civil servant had "sprayed his opinions around" during the referendum campaign.
Mr Straw replied: "The civil service machine is there to serve the public as well as the government of the day and has to abide by very clear principles of balance and impartiality.
"And .. the difficulty,is the machine is committed to take part in referendum campaigns."
Earlier this year MPs on the Commons Public Administration Select Committee (PASC) criticised the publication during the campaign of advice Sir Nicholas gave the Chancellor George Osborne, warning against a currency union with an independent Scotland.
Tommy Sheppard, the SNP's Cabinet Office spokesman, said: "The UK Government was clearly in breach of the purdah rules, and the Treasury under Sir Nicholas Macpherson went rogue and became wholly politicised - his role has already been severely criticised in a PASC report in March.
"In this activity, as well as other matters such as ... secret polling, taxpayers funded the No campaign.
"The purdah rules are there for a reason."
The UK Government denied breaching purdah during the referendum campaign.
A source said that the mail had been sent to Scottish households drop was before the purdah period kicked in.
A Scotland Office spokesman said: "It was a very important decision and people needed to properly informed about what was at stake and the issues behind them.
"It was also a clear government policy to keep Scotland in the UK and we had a duty to clearly explain that to people in Scotland."
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