Van der Westhuizen removed from Grand Slam game
Welsh referee Nigel Owens will now run the line instead of the South African after global governing body World Rugby announced Thursday they took full responsibility for an "oversight".
Van der Westhuizen refereed England's training session at their Bagshot squad base on Tuesday, just five days before he was due to run the line in their match against Ireland at Twickenham.
World Rugby regulations bar match referees – in this case Australia's Angus Gardner – from working with competing teams in the build-up to a Test but no such prohibition exists for their assistants.
But, ahead of a high-profile fixture, World Rugby have acted to avoid any hint of a conflict of interest.
"World Rugby has confirmed that Marius van der Westhuizen will be replaced by Nigel Owens as Assistant Referee Two for the Six nations match between England and Ireland at Twickenham on Saturday," they said in a statement.
"The decision follows Marius's attendance at an England training session this week.
The statement added: "World Rugby takes responsibility for this oversight and has taken the proactive decision to stand Marius down to avoid any additional unfair and unnecessary conjecture.
"Marius is an outstanding talent with a big international future and both he and his employer SARU [South African Rugby Union] fully support the decision."
Ireland coach Joe Schmidt had earlier suggested World Rugby needed to tighten its regulations, saying: "I'm sure in retrospect people are probably thinking it's not the best thing to do."
But he added: "I know Marius and I would have no hesitation in standing by his integrity."
Prior to World Rugby's announcement England coach Eddie Jones defended Van der Westhuizen's involvement with his side by saying: "We went through World Rugby, which is the proper channel."