The founders of London-based hedge fund Parvus Asset Management like to play their cards close to their chest. By showing their hand in a recent investment, they thwarted a proposed merger of gambling companies William Hill Plc and Amaya Inc.

Parvus, set up in 2004 by a former Merrill Lynch fund manager and analyst, operates from a discreet townhouse in London’s Mayfair neighborhood, home to Claridge’s Hotel and the luxury boutiques of Bond Street. It has achieved 15 percent annual returns, according to a person familiar with the situation, at a time when many funds have been struggling to eke out single-digit returns.

The proposed merger began to unravel last week when Parvus, which owns more than 14 percent of U.K. betting-shop operator William Hill, said it was “extremely concerned” by the potential combination with the Canadian owner of the PokerStars online gambling service. In a letter to William Hill’s board that Parvus released publicly, the firm said the deal would destroy shareholder value and urged management to consider a sale instead.

The intervention from a fund that prefers to operate behind the scenes prompted other shareholders to register their opposition with William Hill, which said Tuesday that it had called off merger plans with Amaya. While hedge funds in Europe have sometimes taken a quieter approach than their counterparts in the U.S., shareholder activism has been on the rise in the U.K. since the country’s vote to leave the European Union created buying opportunities by driving down the pound.

‘European Style’

“Their European style of activism has worked well for investors,” said Saleem Siddiqi, founder of Musst Investments LLP, which provides capital to hedge funds. “Parvus’s move on William Hill appears to be prudent.”

It wasn’t the first time that Parvus has upended management plans. In 2011 it led opposition to a 5.2 billion-pound ($6.4 billion) bid from U.K. security outsourcing company G4S Plc for Danish catering provider ISS A/S, calling the proposed combination an “untested vision.” The setback was the beginning of the end for former G4S Chief Executive Officer Nick Buckles, who left in 2013.

The fund, with more than $4 billion under management, primarily bets on Western European companies of all sizes, buying shares and derivatives. It manages long-only as well as hedged funds.

Parvus is an affiliate of TCI Fund Management Ltd., which has a 20 percent voting stake in Parvus, according to regulatory filings. While the two funds share the same building, TCI and founder Chris Hohn don’t have any day-to-day control over Parvus. TCI declined to comment.

Parvus is run by Edoardo Mercadante, a former fund manager at Merrill Lynch Investment Management who was born in Italy, and Mads Gensmann, a Danish former Merrill analyst who also worked at TCI before joining Parvus. They bought into William Hill in 2014.