The chief executive of the rugby players' Association, Damian Hopley, has warned England's stance on not selecting players based abroad could be challenged under European law.
Hopley broadly supports the RFU's move to bring England players of all levels under their control in the build-up to the 2015 World Cup on home soil.
But he is concerned players' earning potential will be hit if they have to turn their back on lucrative contracts with French Top 14 clubs.
Hopley said: "Clearly, we want the best players to be available to play for England but this approach could be seen as somewhat heavy-handed. I understand the intentions behind the approach but there are a lot of people in the game who are nervous about the execution and practice.
"We all have to be realistic about EU legislation and, if there is a restraint on a player, it goes to the heart of freedom of movement for our members. We will be taking legal advice once we see the letter (sent to players by the RFU chief executive John Steele) but I have no doubt if there was an appetite by a player to consider challenging this they would be on strong ground.
"Players will now be forced to make some very big decisions far sooner than some may have anticipated. Clearly everyone wants to play at the top of their capability, which means representing England.
"My personal fear is it will force the hand of quite a few people, but it will ultimately be down to individual choice."
Jonny Wilkinson and James Haskell are both out of contract at the end of this season. If they choose to continue playing in France, they could be ending their international careers.
Similarly, the Sale prop Andrew Sheridan may now think twice about his proposed move to a Top 14 club.
The RFU, who believe the move gives England the best chance of winning the 2015 World Cup, left themselves the option of selecting an overseas-based player under undefined "exceptional circumstances".
And Hopley would not want to see a player mount a legal challenge because he says the RPA recognise, along with the RFU and the clubs, how important a strong Premiership is.
"We want to do what is best for English rugby and we recognise having our best English players based in the Premiership and having strong Heineken Cup squads is best for English rugby and ultimately England.
"But we can't look at it in isolation. Players have to be mindful they are one injury away from ending their careers.
"They need to be maximising their earning potential in an environment that benefits both their rugby and post rugby careers."
Hopley suggested that one longer term solution might be to consider an increase in the salary cap from its current level of £4million, which is around half the amount available to France's Top 14 clubs.Reuse content