Downing Street has told the BBC it uses false names on letters to MPs and members of the public.
The admission came after Labour MP Sir Gerald Kaufman said he got a letter from No 10 signed by a "Mrs E Adams".
But when he called Downing Street he was told she did not exist as the name was "computer-generated", which meant the signature was false.
A No 10 spokesman said real names had not been used on correspondence since 2005 for security reasons.
The policy was adopted after a member of the public traced a member of staff in the Direct Communications Unit, that handles letters from the public and MPs, and threatened her at her home address.
The Downing Street spokesman said: "The security team therefore recommended that staff no longer use their own names, as it was deemed to pose an unacceptable and unnecessary risk to their safety.
"In light of concerns raised in the House (of Commons) today, we will look into alternatives to the use of pseudonyms, but we are clear that our priority is the security of our staff."
Sir Gerald revealed the practice when he raised a point of order in the House of Commons.
He said he had written to the prime minister on 26 April on behalf of a constituent in his Manchester Gorton seat and had received a reply signed with a false name and signature.
On contacting Downing Street he was first told "Mrs Adams" did not speak on the telephone, before another staff member revealed she did not exist.
"I was, first of all, put on to somebody in the correspondence unit who told me that Mrs Adams did not speak on the telephone," he explained.
"I then said that since she had written to me I assumed that she was capable of speaking to me on the telephone.
"I was then put on to somebody who described themselves as head of the correspondence unit who said that Mrs Adams did not exist but was a computer-generated name - and presumably also a computer-generated bogus signature as well."
He added: "What extraordinary events are taking place in 10 Downing Street whereby they send letters from somebody who doesn't exist and expect one to accept this?"
Previous prime ministers had always replied to his letters personally, Sir Gerald stressed.
In response, Commons Speaker John Bercow said it seemed "peculiarly unfortunate" the MP's query had been dealt with in this way.
"I do think it is of the utmost importance that members should be treated with courtesy by the department or agency to which they write," he said.