The startling admission was made in a report called The Defence Communications Strategy, which also marks the government's first official acknowledgement that there is little or no public support for the war in Iraq.
Simon MacDowall, the Ministry of Defence's director general of media and communications, oversaw the report, which he described as an attempt to reduce the bureaucracy in the department's PR operations.
One section of the report reads: "We have no clear idea of the number of people involved in defence communications work or their costs. Over 1,000 people in the MoD have a media/communications job code. This excludes many military personnel involved in communications work."
The MoD's so-called communications department also confesses that it does not know whether its army of PR people is having any effect.
It said: "We need to be able to measure the impact of our communications effort both internally and externally."
Ongoing operations in Iraq were specifically mentioned, including the lack of support for the war, although Afghanistan fared better.
The report stated: "Operations in Iraq are not supported by the majority of the public and operations in Afghanistan are supported by only a narrow majority. There is a lack of public understanding of the rationale behind each mission."
The document also readily endorses the use of "news management" or "spin" to influence public opinion.
This is best achieved, it says, by "creating a steady stream of positive stories which directly promote the MoD and forces' reputation, but also helps to offset the inevitable bad stories".
The MoD said yesterday that the seventh British serviceman to die in Iraq this month was killed in a mortar attack on a British base in Basra. He was named as Lance Corporal Timothy Darren Flowers of the Royal Electrical and Mechanical Engineers.
The British death toll stands at 163.