Mr Portillo's office overlooks Horse Guards Parade, and ministers usually hold parties so that their guests can watch, but members of the public attending the ceremony on Wednesday night complained about disruption from "raucous voices" and "loud laughter" through the open windows.
There was "a request that the people standing near the window keep their voices down and the request was complied with", a spokeswoman for Mr Portillo said. "He is very sorry that anybody was disturbed. He did something about it right away."
But Mr Portillo's supporters said they thought the story was "wildly exaggerated" and was being used by his political opponents, or by those in the armed forces who did not like his Tory conference speech last year, in which he invoked the reputation of the SAS.
Mr Portillo was overtaken in a Mori opinion poll this week by John Redwood, the former Secretary of State for Wales, as the leading right-wing contender for the Conservative Party leadership if Labour wins the next election.
He was also the victim earlier this year of unfounded suggestions that he wanted to sell off the historic Admiralty Arch connecting Trafalgar Square to the Mall. It was reported that John Major had slapped him down before it emerged that the property had been handed over to the Department of the Environment some time before.
The London Evening Standard yesterday quoted an unnamed "former Guards officer" as saying that Mr Portillo's party "utterly ruined an emotional and stirring occasion which a lot of people had come to see. You would think Portillo would know better". A source close to Mr Portillo said: "When you are a politician there are always people trying to do you down. He can cope with it."Reuse content