The PA news agency was told that Lulu Lytle, the interior designer behind lavish renovations of his No 10 flat, briefly attended while undertaking work in Downing Street.
A No 10 spokeswoman said: "A group of staff working in No 10 that day gathered briefly in the Cabinet Room after a meeting to wish the Prime Minister a happy birthday. He was there for less than ten minutes."
ITV news also reported that later that evening family friends were hosted upstairs in the Prime Minister's official residence.
Downing Street said: "This is totally untrue. In line with the rules at the time the Prime Minister hosted a small number of family members outside that evening."
A spokeswoman for Soane Britain, the luxury designer co-founded by Ms Lytle, said: "Lulu was present in Downing Street on June 19 working on the refurbishment.
"Lulu was not invited to any birthday celebrations for the Prime Minister as a guest.
"Lulu entered the Cabinet Room briefly as requested, while waiting to speak with the Prime Minister."