Harlequins have launched an investigation after claiming many of their supporters sold tickets on to Munster fans for last Sunday's Heineken Cup quarter-final at the Twickenham Stoop.

Under Heineken Cup participation rules, two-time European champions Munster were allocated 25% (3,750) of available tickets for the game, which attracted a ground record 15,000 crowd.

But in a strongly worded statement published on the club's website today, Quins said it was "obvious from 90 minutes before kick-off" that Munster supporter numbers "exceeded" their ticket allocation.

Munster fans roared their team to an 18-12 success, setting up a semi-final appointment with French challengers Clermont Auvergne in Montpellier later this month.

"In the two weeks leading up to the match, the club was aware that tickets were being sold on Ebay and through other unauthorised channels," Quins said.

Harlequins says it tried to identify the source of the tickets in these cases and attempted to cancel them.

"Last Sunday, it was obvious from 90 minutes before kick-off that Munster's well organised and wonderful supporters had a presence in the stadium that exceeded the number of tickets officially allocated to them.

"The colourful and vocal support they gave their talented team during the match provided further evidence of this, if it was needed.

"Given the controlled manner that tickets were made available for the match, it is clear to the club that many of our 'supporters' sold their tickets on to opposition fans.

"To say that this is disappointing for the players, coaches, management and staff of the club is an understatement."

The club says it is determined to identify any person that sold on their tickets and "will ensure that last Sunday's outcome is not repeated."

Reigning Aviva Premiership champions Quins return to The Stoop on Saturday for a league appointment with Bath.

And the club clearly feel let down that tickets for such a high-profile occasion were sold on as Quins strived to reach their first Heineken semi-final.

"The club ... is very disappointed that a bigger share of those present were obviously not Harlequins' supporters, despite the club's best efforts to ensure that they had priority in the ticketing process," the statement continued.

"In deciding to host such a high-profile occasion with such experienced and prestigious opponents as Munster, the club recognised from the outset that its ticketing policy had to be transparent and that it would be put to the test, given the anticipated demand.

The club says no tickets were on sale to the general public.

"Instead, Harlequins season ticket holders and members were given priority access to the 8,520 tickets that were available for them. All of these tickets were rapidly taken up within the restricted times they were on sale.

"Further tickets (750) were allocated as required by the tournament rules to the organisers ERC (European Rugby Cup) and its sponsors (Heineken, Amlin, EDF, FedEx and adidas).

"In addition, a number were used for Harlequins' sponsors, corporate guests, community clubs, players, staff and official charities. The few tickets that were returned were sold to the club's season ticket holders."