Warren Gatland said on Tuesday that English Premiership clubs could scupper their players' chances of representing the British and Irish Lions in South Africa if they fail to release them before a warm-up match.
Lions head coach Gatland accepts he may not have a full squad for the match against Japan at Murrayfield on 26 June - the same day as the Premiership final.
Yet, even players at Premiership clubs not involved in the final could be unavailable for the match in Edinburgh, as it is scheduled outside World Rugby's window for international games.
Gatland, however, wants to maximise his time with the squad given the July-August tour has already been shortened. There will be at least two fewer games than on the trip to his native New Zealand in 2017, when he guided the side to a drawn series against the All Blacks.
If a compromise, which could involve cash payments from the Lions to Premiership clubs, cannot be found it won't only be England players whose chances are at risk. Scotland captain Stuart Hogg and Wales fly-half Dan Biggar, who play for Exeter and Northampton respectively, could miss out.
"It's not just the English players," Gatland said.
"We understand about the Premiership final and players in that, but we hope to be together for a couple of weeks of preparation leading into the warm-up game against Japan in Edinburgh."
Former Wales coach Gatland estimated between 16-20 members of his squad could come from the Premiership.
"If some of those players are not available for that preparation time... unfortunately some of those players may miss out.
"I think the Lions are talking to PRL (Premiership Rugby Limited) this week and I'm just hoping we'll get some resolution."
England finished fifth in this year's Six Nations, with captain Owen Farrell and Maro Itoje - once regarded as nailed-on certainties for the Lions - not at their best.
"It's probably not like it was 12 months ago where some of them would have been the first names on the sheet," Gatland said.
'Scots fronted up'
Scotland coach Gregor Townsend and Dark Blues defence chief Steve Tandy are both among Gatland's backroom staff.
It is 20 years since a Scotland player - prop Tom Smith in Australia - last started a Lions Test.
But in this year's Six Nations Scotland ended long waits for away wins against England and France, with fly-half Finn Russell and flanker Hamish Watson impressing.
"There will definitely be more Scots this time because they have fronted up in the Six Nations," Gatland said.
"Winning away in Paris and at Twickenham is significant because when you are playing for the Lions you are not playing at home."
Gatland, denying accusations of anti-Scottish bias, added: "Four years ago what was probably damaging to the Scottish players is that you go down to Twickenham and you get 50 points put on you. That's not a good look."
South Africa have not played a Test since beating England in the 2019 Rugby World Cup final in Japan as a result of the coronavirus pandemic.
Covid-19 concerns also led to fears the Lions would call off their tour.
Matches could yet be played behind closed doors, while the eight-game schedule, culminating with three Tests, may be altered.
But Gatland warned Rassie Erasmus' world champions, who hope to play warm-up matches, would still be formidable opponents.
"South Africa were outstanding and got better as the World Cup went on," Gatland said.
"Rassie's gone back to their DNA in terms of their physicality. We've got to be mindful of that. Whatever South Africa team we play is going to be tough.
"Playing against the world champions is incredibly exciting and also a massive challenge."