The 23 year old Royals keeper will move to Fratton Park on Wednesday when his contract expires, but as he is under 24 the Football LEague Championship side are entitled to a fee.
Reading are said to be holding out for £400,000, with Portsmouth's valuation, as it always tends to be in these cases, considerably lower.
Reading confirmed that they intend to take this issue to tribunal, likely to be seen in late July.