UKIP climate change spokesman and deputy leader Viscount Monckton has been warned to stop calling himself a member of the House of Lords.
It follows an interview he gave to Australian radio in which he said he was a member "but without the right to sit or vote".
But he was told not to repeat the claim by the Clerk of Parliaments.
In a letter David Beamish told him: "you are not and have never been a Member of the House of Lords".
Legislation in 1999 ended the link between holding a hereditary peerage and being a member of the Lords.
Viscount Monckton inherited his title from his father who died in 2006 - who had been a member of the House of Lords until 1999, when the legislation was introduced.
He has since stood in four by-elections to replace hereditary peers who had died - but failed to be elected.
'Contradiction in terms'
In his letter the Clerk of Parliaments says his predecessor also wrote to Viscount Monckton twice, in July 2010, asking him to stop claiming to be a member of the House "either directly or by implication".
But he says the peer has since continued to make the claims.
"In particular, I have listened to your recent interview with Mr Adam Spencer on Australian radio. In response to the direct question, whether or not you were a Member of the House of Lords, you said "Yes, but without the right to sit or vote". You later repeated, "I am a Member of the House"," Mr Beamish wrote.
"I must repeat my predecessor's statement that you are not and have never been a Member of the House of Lords. Your assertion that you are a member, but without the right to sit or vote, is a contradiction in terms.
"No-one denies that you are, by virtue of your letters patent, a peer. That is an entirely separate issue to membership of the House."
He goes on to add that he is publishing the letter "so that anybody who wishes to check whether you are a Member of the House of Lords can view this official confirmation that you are not".
Viscount Monckton, who served as an adviser to the Thatcher government between 1982 and 1986, is a former journalist and is well known as a climate change sceptic.
He joined the UK Independence Party in 2009, as its chief spokesman on climate change. He became the party's deputy leader last year.