Joost van der Westhuizen has launched a scathing attack on Heat magazine in a lawyer's letter to the publication's legal team.

The document is in Eyewitness News' possession.

On Tuesday, Heat hosted a media viewing of a controversial sex tape it claims features the former Springbok captain.

In the letter delivered to Heat's attorneys, Van der Westhuizen's lawyers questioned the motives of the publication.

They claimed Heat was intentionally hiding the source of the video who — in their opinion — was guilty of criminal conduct.

The celebrity sportsman's attorneys said further attempts to publicise the footage was unlawful and was part of a conspiracy to assassinate his character and hurt his family.

They believe the tape was fabricated and edited.

They claim it was for this reason that Heat would not allow van der Westhuizen to have technical analysis done on it.