Martyn Williams says Welsh players "will have issues going forward" if they chose to play for English clubs.
Premiership Rugby will not allow their clubs to release players for training and matches outside World Rugby's international windows.
Wales centre Jamie Roberts has joined Harlequins and Taulupe Faletau is being linked with a move to Bath.
"Players and their agents have got to sit down and think carefully," said former flanker Williams.
Wales regularly play at least one autumn Test and also hold training camps outside the governing body's allocated time slots.
Premiership Rugby - the body that represents the major English clubs - has an agreement with the RFU to release players for additional England duty.
Williams, who won 100 caps for his country, is warning players to be aware of the situation before they commit to a move.
He said: "Ideally we want all our best players playing in Wales there's no doubt about that, signing dual contracts so they're under control of the WRU and the Welsh management team.
"But economic forces are going to suggest that players are going to look outside to France and England and it will have issues going forward definitely.
"Hopefully we're not going to get a scenario where [England winger] David Strettle for instance has chosen his club over his country.
"For me it's always country first, that's the be all and end all."
Williams believes Roberts and Faletau would have checked things before opting to move, but thinks agents play a big role.
"Hopefully it'll put players off going abroad or going to England because we want our regions as strong as we can as well as the national team," Williams added.