Reigning 250cc world champion Marco Simoncelli has confirmed that he will make his World Superbike debut, riding Aprilia's RSV4 alongside Max Biaggi, in this weekend's Imola round.

Rumours that Simoncelli would make a shock race appearance were triggered by a test session on the V4 four-stroke at Mugello on Tuesday and Wednesday, in which fellow 250GP star Mattia Pasini.

Simoncelli - who rides for the Aprilia-owned Gilera brand in grand prix - first rode the WSBK machine during a test at Valencia in late 2008, spawning rumours that he might enter the opening rounds of this season.

Those rumours came to nothing, but Simoncelli's competitive pace at the Mugello test - combined with a gap in the grand prix calendar, an injury for regular rider Shinya Nakano and Simoncelli's slim 2009 250 title hopes - have suddenly made a WSBK debut possible.

Simoncelli set a best lap of 1min 53.55sec during 91 test laps, compared with a 1min 53.4 for Biaggi (Wednesday), a 1min 52.3sec for Ducati's title contender Noriyuki Haga (Wednesday) and a 1min 53.5sec for his team-mate Michel Fabrizio (Tuesday).

"I am very satisfied with this test," said Simoncelli. "I was quite quick and together with the bike managed to improve a lot.

"We worked to find the right base setting for Imola, where it will be important to find a good set-up as quick as possible, which will make things much easier.

"I'd like to thank Aprilia for this great opportunity, I will be making my debut in a category and on a track that are completely unknown to me, the level of our rivals is very high and it won't be at all easy to obtain a good result."

Biaggi welcomed his one-off team-mate.

"I am very pleased for Marco," reports Biaggi as saying. "Simoncelli is welcome and it will be good to have him in the team. I think it is a good opportunity for him. I hope Nakano will soon recover from the neck injury he suffered in Germany."

Winless during his first two years in 250GP, the 22-year-old has won ten races and taken 19 podiums since the start of his title winning 2008 season, although a mistake at Misano looks to have put this year's crown out of reach. Simoncelli sits third in the standings, 40 points behind Hiroshi Aoyama with four rounds to go.

Like many of Aprilia's past 250cc stars, Simoncelli will switch to Honda for his 2010 MotoGP debut, but the Italian factory can at least enjoy a formidable line-up of former four-time 250cc champion Biaggi and present champion Simoncelli - on home asphalt - for one weekend in WSBK.

Biaggi has taken the brand new RSV4 to five podiums, including one win, so far this season and sits fifth in the world championship standings with three rounds to go.

Simoncelli and WSBK title leader Ben Spies could be set to race each other for rookie of the year honours in MotoGP next season, should rumours of the Texan's Tech 3 deal be confirmed.