UPDATE: Row erupts between England and NZ over Shields
This follows Eddie Jones's request to include Hurricanes forward Brad Shields in the England squad for the three-Test tour of South Africa in June.
RugbyPass has been told that with New Zealand insisting they have the right to block Shields' release and the Rugby Football Union adamant the player must be allowed to tour under Regulation Nine.
It will be up to World Rugby to broker a peace deal.
English chiefs are convinced Shields will have a Regulation Nine release clause in his contract and are seeking clarification.
Shields has signed a lucrative contract to join Wasps and is due to arrive after the Hurricanes' Super Rugby campaign and is qualified to play for England through his parents.
It is clear that with England dealing with back row injury problems which have ruled Northampton's Courtney Lawes and Wasps Nathan Hughes out of the tour, they see Shields as the obvious replacement despite the forward still being involved with the Hurricanes.
It also raises the prospect of Shields being in the frame to play for England in the much-anticipated clash with the All Blacks at Twickenham in November.
Last summer Piers Francis was released by the Blues to take part in England's summer tour to Argentina but it is understood he only had a contract with his Super Rugby franchise, not the New Zealand RFU. Shields is different which means Steve Tew, the New Zealand chief executive, is making it clear the player will not be released for England.
World Rugby's Regulation Nine states that all players must be released from club commitments during the agreed June and November international windows. New Zealand Rugby doesn't" want to let him go and believe they have the legal right to deny release.
Tew said: "I don't think you should jump to the conclusion that he will be available from our point of view. He has signed to New Zealand and he is contracted to play for New Zealand teams until the end of Super Rugby.
"We have a New Zealand player who is contracted to be here until the end of that competition and that would be our expectation. We are obligated to release players who have signed to play for other countries so they have made themselves unavailable for New Zealand.
"We always make sure that occurs and that is of particular relevance to the Pacific countries. But in this instance, Brad has signed a contract that makes him available for New Zealand teams.”