Players contracted to Major League Soccer have voted to go on strike if no new labour agreement can be agreed with the league before the season is due to open on March 25.

The players union and the league have been locked in tense negotiations for several weeks, with the previous deal having expired on January 31, then twice extended until February 25. The union refused further extensions.

Now the union has confirmed what some players had already suggested - that they will not start the season without a deal in place.

"Recent comments from players simply reflect the fact that the players are unified and, per the results of our strike vote, will not begin the new season if a new agreement with the league is not reached," union executive director Bob Foose said.

"This is not a change in position by the union and should not be read to reflect in any way upon what has, or has not, occurred this week in the meetings with the mediator and the league."