British & Irish Lions coach Warren Gatland has warned players who miss the team's warm-up fixture against Japan on June 26 that they will be at a disadvantage when he makes his final selection for the tour of world champions South Africa.
The Lions are locked in a battle with Premiership Rugby over the release of players with the competition final to be staged on the same weekend, which falls outside of World Rugby's Regulation 9 period that forces clubs to release players.
Gatland has revealed the Lions are also planning a camp in Wales ahead of the Japan game, which will be important in formulating his selection plans.
"It's not 100% final at the moment [that players will not be released], there are still some negotiations that will go on between now and then," Gatland told reporters on Wednesday.
"But if England have a great Six Nations it could mean that a third of the players [in the Lions squad] are missing for the Japan game and a camp in Wales the week before.
"We want to take the best players on tour, but the reality is that it could effect selection. If you end up with a 50-50 call on a player, the players who can be at the camp in Wales and the Japan game might get the nod over someone who is not available until the week before the first game in South Africa."
Gatland has suggested the England-based players should petition their clubs to be released.
"Hopefully common sense prevails, to go on a Lions tour and the interest it creates from within and outside the sport, you can come back a superstar. Maybe the players need to voice their opinion on that, it would be pretty powerful I think," he said.
Only two of the 12 English top-flight clubs will be involved in the final, but Premiership Rugby has said none will be releasing players ahead of the Regulation 9 window, including Saracens, who will be competing in the Championship.
The Lions will play three Tests and five warm-up games on the tour that runs from July 3–August 7.