The 20-year-old had already revealed he was set for a switch to The Hawthorns and the two clubs have now confirmed the deal.

Simpson, who spent last season on loan at Millwall, made his Arsenal debut in the Carling Cup against Sheffield United earlier this season.

He then scored twice against Wigan Athletic in the same competition before featuring in the 2-0 defeat at Burnley in the quarter-finals.

However, he has yet to make a Premier League appearance for Arsenal and Baggies boss Tony Mowbray admitted he is taking a gamble on the youngster.

"I would suggest Jay is an unknown quantity at Premier League level," conceded Mowbray recently. "Can he score goals? I don't know.

"But we'll hopefully give him an opportunity to see if he can.

"He's obviously had a very good upbringing at Arsenal and every player needs an opportunity."

Simpson signed a new long-term contract at Arsenal on Monday.