Volkswagen's Audi division said it agreed to buy Italian motorcycle maker Ducati, adding a 12th brand to the German auto maker's portfolio of passenger cars, trucks and ultra-luxury vehicles.

The 20-member supervisory board of Europe's largest car maker approved the purchase on Wednesday, Audi said in a statement. It did not disclose details of the deal, though sources told Reuters on April 17 that Audi agreed to buy Ducati for about 860 million euros (705 million pounds).

Ducati, which has won 13 rider's Superbike World Championships since 1988, will extend Audi's long-standing rivalry with BMW to superbikes.

As a sporty, global premium brand, Ducati is an excellent fit for Audi, the company's chief Rupert Stadler said.

Audi outsold Daimler's Mercedes-Benz last year and has vowed to surpass BMW for the top spot in luxury car sales this decade.

Audi said it aimed to complete the purchase of Ducati from Italy's Investindustrial as quickly as possible once it has received approval by the relevant regulators.

Analysts have questioned the Ducati purchase, saying the move bears no economic or industrial logic and solely reflects VW Chairman Ferdinand Piech's passion for the Italian company's expertise on design and light engines.

I can't think of a concrete reason for Audi to warrant a business case for buying Ducati, said Stefan Bratzel, director of the Center of Automotive at the University of Applied Sciences in Bergisch-Gladbach.

Ducati doesn't enhance Audi's business model in any way, it's just a trophy in the wall cabinet, he added.

Ducati also brings in liabilities of well below 200 million euros, one official told Reuters.

Ducati makes about 40,000 two-wheelers a year across its range of cruiser, supermoto, adventure, naked and superbikes that include the muscular Diavel and dual sport Hypermotard. By comparison, industry leader Honda <7267.T> sold over 16 million motorcycles last year.

Investindustrial bought Ducati in 2006 and has since grown the motorbike maker's annual revenues by 37 percent to 480 million euros. Ducati had earnings before interest, tax, depreciation and amortisation of 51 million euros last year.

We believe that Audi is the best partner to continue the process of globalisation that is already successfully underway, Andrea Bonomi, chairman of Investindustrial, said.

Under Piech, who looks set to win re-election as chairman at VW's annual general meeting in Hamburg on Thursday, VW expanded its empire to include everything from fuel-efficient city cars to 40-tonne trucks.

The Wolfsburg-based car maker is currently looking for ways to buy a remainder of 50.1 percent of sports-car maker Porsche
and said on April 13 that it further raised its majority stake in truck maker MAN .

(Reporting By Andreas Cremer; Additional reporting by Jennifer Clark in Milan, editing by Dave Zimmerman)