Liberal Democrat Chief Whip Paul Burstow breached parliamentary rules by sending letters and newsletters to constituents in pre-paid House of Commons envelopes funded by the taxpayer, Westminster's Standards Commissioner ruled yesterday.

Mr Burstow has apologised and offered to repay about £1500 to cover the cost of the stationery and postage, and Commissioner John Lyon said he now regarded the matter as "closed".

Conservative MP Crispin Blunt made a complaint to Mr Lyon after hearing that letters to new constituents sent in pre-paid Commons envelopes contained party political attacks on the Sutton and Cheam MP's Tory rival in the next general election.

Mr Blunt yesterday said that Mr Burstow had been "deliberately misusing the system to gain an unfair political advantage over his opponent at the next election at taxpayers' expense".

But Mr Burstow insisted that it was "a simple administrative error which shouldn't have happened and won't happen again".

Mr Lyon ruled that the material sent between January 2007 and June this year should not have been sent in pre-paid Commons envelopes, which are intended for replies to constituents on issues on which they have contacted their MP.

But he said the LibDem MP could legitimately have charged the cost of sending the letters against his parliamentary allowances.

In a letter to Mr Blunt, the Standards Commissioner wrote: "I have accepted the actions which Mr Burstow has taken in response to his breach of the rules and to rectify the matter. He has apologised. I therefore regard the complaint as now closed."

Mr Lyon said he would report his findings "informally" to the cross-party standards and privileges committee, which oversees the handling of allegations against MPs, but would not be recommending it should take any action.

Reigate MP Mr Blunt said: "At a time when the public are deeply sceptical about how MPs use taxpayers' money to do work on behalf of their constituents it is to be regretted that the Liberal Democrats' own chief whip was deliberately misusing the system to gain an unfair political advantage over his opponent at the next election at taxpayers' expense.

"I believe this practice directly undermines democracy by the incumbent politician unfairly entrenching this position. This must stop. It strikes straight at the heart of an open and fair democratic process."

Mr Burstow said: "Mr Lyon's letter rightly says that I acted very quickly once the error had been made in my office resulting in the letter I was sending to new constituents being put in pre-paid envelopes. As soon as I discovered what was happening, I stopped it.

"Mr Lyon is not taking this to the standards and privileges committee. I think people should draw a conclusion from that."