Eddie Jones defended his right to be "contradictory" after naming Danny Cipriani in his England squad for the tour of South Africa next month.
Dylan Hartley will miss the tour because of concussion so Owen Farrell will be England captain for the three-Test series starting in Johannesburg on 9 June.
"We're missing 20 players who would be in the top 45 of the players in England so it's a great opportunity for England to expand its depth, it's a great opportunity for us to do something that's never been done before", said Jones. Including Shields there are eight uncapped players in total while James Haskell, Danny Care and Dan Cole are among the players omitted.
Hurricanes captain Brad Shields scores a try against the Sunwolves.
"We have taken some time to carefully work through the complexities of this request‚ to ensure that we gave appropriate consideration to his specific circumstances‚" NZR head of professional rugby Chris Lendrum said.
Lendrum said: "We are extremely disappointed that the RFU chose to take this unusual step in seeking this release given that Brad has not yet played rugby in England". He last started for England in 2008 but his selection can be considered reward for a fine finish to the season, including a man-of-the-match performance for Wasps against Newcastle last week.
In all, seven players have been called up by England for the first time - Ben Earl (Saracens), Jonny Hill (Exeter Chiefs), Dan Robson (Wasps), Brad Shields (Hurricanes/Wasps), Ben Spencer (Saracens), Cameron Redpath (Sale Sharks) and Jack Willis (Wasps) are all in. Newcastle Falcons' Mark Wilson also returns after a year on the global sidelines, while Bully Vunipola is also in after his recent injury problems. Scott Wisemantel will join Jones' coaching staff for the tour to South Africa as an attack consultant.
'There are a number of players out injured or not selected so it provides a big opportunity for others'.
"We have an opportunity to create some history next month and win the Test series, something no England side has done before".
England will play South Africa in three Tests in Johannesburg (June 9)‚ Bloemfontein (June 16) and Cape Town (June 23).