The 18-year-old winger, who is the brother of Socceroo Rhys, has been signed by the Cottagers for an undisclosed fee, although the Daily Mail claimed it was worth £500,000.

Williams has made six first-team appearances for Pompey this term, but has been offloaded as the cash-strapped club tries to raise funds.

Pompey boss Michael Appleton said: “This is a good opportunity for Ryan to move to an established Premier League outfit and we wish him all the best for the future.”

Perth-born Williams had already travelled to Fulham earlier this week to have a medical with the club and was part of their reserves side in Monday's 1-0 win over Wolverhampton.

Williams joined Portsmouth's academy in July 2010 after coming through the ranks at ECU Joondalup in Perth.