Tottenham forward Gareth Bale was sent a death threat on Sunday, according to the Daily Star.
A brief telephone message was received by the Spurs training ground in Middlesex - and it is being taken seriously by the authorities.
Bale, who has been strongly linked with a world-record move to Real Madrid in recent weeks, is believed to be determined to leave White Hart Lane before the end of the current transfer window.
The 24-year-old was not involved in Spurs' 5-2 pre-season friendly defeat to AS Monaco at the weekend, further fuelling speculation that a move to the Bernabeu is imminent.
But Tottenham boss Andre Villas-Boas refuted those reports after the Monaco loss, telling reporters: "These rumours of an imminent transfer are not true. The only thing we have communicated with Real Madrid is that he is our player and he is not for sale."
However, with the player eager to leave and Real Madrid desperate to make him their marquee signing this summer, Tottenham are expected to sell if they receive a bid which equals their valuation of the forward.
Spurs have already received an £85m bid from the nine-time European champions - but that figure was not enough to allow Bale to begin negotiating personal terms with the Spanish giants.