The first minister should refer himself to an independent investigation over his use of inaccurate information about an AM's contacts with a health board, the member concerned has said.
Carwyn Jones prompted a row with his answer to Adam Price's question last week about a proposed hospital shake-up in west Wales by the Hywel Dda board.
Mr Price claims civil servants' time was used to dig up "dirt" on him.
Welsh Government said Mr Jones had committed to correct the record.
The first minister had alleged Mr Price failed to respond to requests for contacts with the health board - comments in the Senedd that led to accusations that data protection law had been breached.
The exchange during First Minister's Questions last Tuesday caused a furore - but Hywel Dda later apologised to Mr Price for giving information to the Welsh Government that was incorrect.
"I am writing to ask for a full apology in relation to statements made by you and by your government defaming me last week and for an independent investigation under the terms of the ministerial code into the illegitimate use of executive power to traduce an opposition member," Mr Price wrote in a letter to the first minister.
The Carmarthen East and Dinefwr AM alleged that "official resources were used illegitimately to access non-public domain information about me in order to denigrate my integrity and my professionalism".
Mr Price said it may have been a breach of the ministerial code, calling for Mr Jones to refer himself to his independent adviser on the code, James Hamilton.
"I regard this as a deliberate attempt to smear me," he said.
Mr Hamilton is already investigating whether Mr Jones misled the Senedd on a different issue.
The letter said: "I am told that the Health Board received a request from your Government for information specifically about me (along with two other questions of a more general nature) at 9.54am on the 26th January.
"It was marked ASAP and related directly to the oral question which I was due to ask you on the following Tuesday."
"The Health Board have told me they had no expectation this information would be placed in the public domain, and would have checked the information with me had this been the case."
Mr Price, who demanded a "full apology", said: "What is expressly not permitted under Civil Service rules is the gathering of potentially damaging information about a Member which may be used to attack the Member in question.
"That breaches the rules of the civil service code on impartiality which expressly prohibit the 'use of official resources for party political purposes'."
The AM asked Mr Jones to acknowledge he had "breached the duty under the Ministerial Code to give a truthful and accurate account, specifically in this case misleading the Assembly" and to "set the record straight through an urgent statement at tomorrow's First Minister's Questions".
The letter also alleges that a media statement issued by a spokesman for the first minister was defamatory of Mr Price.
"This statement again alleges that I have misled the public in not actually seeking to engage with the health board and that I am a hypocrite as a consequence", the Plaid AM said.
A spokeswoman for the Welsh Government said: "The first minister has already committed to correcting the record."