Scandinavian activists in the Gaza-bound flotilla say someone sabotaged their ship's propeller while the vessel was docked in a Greek port.
The Swedish activists say the propeller shaft of the "Juliano" was cut off in Piraeus and that it would take a couple of days to repair the damage.
The ship is shared by Swedish, Norwegian and Greek activists and is part of a 10-ship flotilla due to set sail shortly in a bid to break Israel's naval blockade of Gaza.
Israel says it will block any attempt to breach the naval blockade of the Palestinian territory.
Swedish activists said the propeller damage was discovered late Monday. Norwegian delegation leader Torstein Dahle said on Tuesday that "someone is willing to go to great lengths to stop the flotilla from sailing."