James Haskell is reconsidering his planned move to Stade Francais
England's players' union is taking legal advice about a letter warning stars they could jeopardise their international places by moving abroad.
The four-page letter, signed by Rugby Football Union chief Francis Baron, was sent to all members of Martin Johnson's elite squad last week.
Damian Hopley, head of the Professional Rugby Players' Association, told BBC Sport: "We are taking legal advice.
"The concern is that this becomes a restraint of trade."
The letter was sent in the wake of three England players - James Haskell, Riki Flutey and Tom Palmer - announcing last month that they would be moving to France at the end of the season.
Although the letter states any player moving abroad will not be "banned" from the England squad, it makes it clear they could jeopardise their places if satisfactory release periods are not written into their contracts.
While it was not part of the content of the letter, players have been left in no doubt that if they move abroad and are vying for a place with an English-based player of the same ability, they will not be selected.
It has also been made clear that Johnson and his coaching staff will not be travelling to watch players in the French league.
"On one hand the RFU says it's fine for players to go to France, on the other that it would not be as attractive," he said.
"You have to remember that it's not just a financial decision for players to go, but also about lifestyle and experience.
"We were proactive in seeking legal advice once we found out about the letter."
Haskell is having second thoughts about moving to Stade Francais now he knows it could make it more difficult to get a place in England's starting XV, BBC Sport understands.
The 23-year-old Wasps flanker lost his starting slot to Tom Croft for last weekend's game against France at Twickenham and has lost out again for England's Six Nations closer against Scotland.
Haskell has signed a pre-contract agreement with the French side, so would face paying money back to them if he decides to stay in England.
Haskell's Wasps and England colleague Palmer has also agreed to move to Stade at the end of the season, while centre Flutey - one of the brightest stars of England's Six Nations campaign - will switch to Brive.
Fly-half Andy Goode moved to Brive at the end of last season, but was drafted into Johnson's squad before the game against Italy, which he started.