The England midfielder’s switch will be completed when his World Cup campaign is over.
The 41-year-old cast doubt over his participation in the one-lap mini race ahead of the Silverstone showpiece.
Clark called last year’s incident at Oakmont the “lowest point” of his career.
Van Poortvliet has edged Alex Mitchell.
The Lancashire seamer impressed in Wednesday’s Vitality series opener against India, which was washed out.
‘It looks a proper Group One and whatever wins it will have to be a very, very good horse’.