Viacom has been attempting to convince civil rights groups that its agreement with YouTube not to disclose user data is genuine.

The company sent a letter to the Electronic Frontier Foundation (EFF) on 16 July regarding a stipulation in its lawsuit against the video-sharing site.

The stipulation states that the YouTube log files that Google is being forced to hand over as evidence will be purged of any defining user names or IP addresses.

The EFF had insisted that the agreement could be changed at any time without warning. In order to allay those fears, Viacom has agreed to warn the EFF in advance should the stipulation be changed.

"We anticipate that the information will be treated as highly confidential under the protective order in the case and available for council eyes only," the company said.

"We believe that these measures will achieve an unprecedented protection of privacy interests."

EFF senior staff attorney Kurt Opsahl said that the letter "helps assuage a huge concern".

"A stipulation is not as protective as a motion to correct the court's order because it may be modified by another stipulation between the parties," he said.

"Viacom's promise, however, gives us the opportunity to take action before personally identifiable information is revealed."