"Dear Mr Pa" was Downing Street's response to Tony Blair's proud father when he wrote to congratulate his son on becoming prime minister.
The below-par performance came soon after Mr Blair won power in 1997 and was revealed by ex-No 10 communications chief Tim Allan.
Leo Blair wrote to No 10 wishing his son the best for his new job - signing off the letter "your loving pa".
No 10 replied to "Mr L Pa" suggesting he contact his citizens' advice bureau.
The 'mad' pile
Mr Allan recalls the episode for BBC Radio 4's Look Back At Power programme.
He recalled how Mr Blair Sr had written saying how proud he was of his son.
Mr Allan explained: "The letters that are dealt with in Downing Street are basically divided into letters the prime minister has to see, letters that have to be dealt with by the office, and long hand-written scrawls from mad people which is by far the biggest pile.
"And so this letter was looked at quickly, you know, cursorily read. But it couldn't really be deciphered, much of the handwriting.
"And so, about a week after he sent it, Tony Blair's father was at home and a letter arrived through the letter box. And it was addressed to Mr L. Pa.
"It said: 'Dear Mr Pa. Thank you for your letter to the prime minister, the contents of which have been noted. Unfortunately, we cannot take up your suggestion but we suggest you contact your local MP or Citizens Advice Bureau.'"
Look Back at Power, a two-part series on BBC Radio 4 about working life in Downing St under Tony Blair, is being broadcast at 1100 BST on 5 September and at 1100 on 12 September.
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