England head coach Stuart Lancaster will live to regret his decision to not pick flanker Steffon Armitage for the Rugby World Cup, according to former New Zealand forward Ali Williams.

Armitage has helped French side Toulon to successive European Champions Cup winners trophies and is regarded as one of the best number sevens around by Williams.

However, as he plays club rugby in France he is unable to represent England unless Lancaster invokes the exceptional circumstances clause.

Williams is quoted as saying by The Telegraph: "When you look at the structure of the game of rugby these days where are the most potent attacking opportunities? It's off turnover ball. If you don't have someone who can produce turnover ball then you are losing a huge attacking as well as defensive strength.

"That's why I don't understand how England can afford to leave Armitage out. Look at what Australia are doing by trying to get David Pocock and Michael Hooper in the same team.

"England have available the best openside in the business after Richie [McCaw] and they say 'no thanks'. He has been in incredible form year after year. Any other side and he would be there."

Lancaster will name his final squad on Thursday.