Karl McCartney, Conservative MP for Lincoln, told one official they were a "pedantic SOAB".
The jibes emerged as Mr McCartney claimed he was a victim of "bullying" by the Independent Parliamentary Standards Authority (Ipsa).
Speaking in the House of Commons, he admitted his comments were inappropriate and apologised.
During Business Questions earlier, Mr McCartney said Ipsa chief executive Andrew McDonald had used "bullying tactics and threats" to silence him regarding concerns he had raised over their spiralling costs.
He then accused senior management of using "false innuendo and subterfuge" to smear the name of politicians.
In a letter later sent to the MP, Mr McDonald denied the bullying allegations and insisted Ipsa was providing good value to the taxpayer.
He wrote: "Some of the notes written by you, and attached to your claims, were recently brought to my attention.
"Having reviewed those notes, I was taken aback by the content, which ranged from abusive through to the offensive and condescending.
"We will do all that is reasonable to shield our team from such treatment. Ipsa's team deserves to be treated in a courteous manner.
"I ask that you desist from correspondence which falls below this standard."
In a statement, Mr McCartney admitted his messages had been "inappropriate" and said he regretted having made the comments.
"I accept that such comments have given cause for offence," he said.
"You will not see me making similar remarks in the future in respect of Ipsa, which has a difficult and important job to do."
An MP has apologised after he wrote offensive notes to his staff processing his expenses claims.