Endeavour Capital is suffering from an extremely severe case of the Mondays. The London hedge fund plummeted 27% on Monday, burned by so-called “box trades” in Japanese government bonds, according to published reports.

The once US$2.9 billion firm wasn’t the only hedge fund loser on the trade—which bets on a simultaneous widening of 20-year spreads and narrowing of seven-year spreads—but it is thought to be the biggest.

Endeavor’s box trade was highly-levered, and the loss forced it to effectively eliminate its leverage, once as high as 18-times.

Endeavor CEO Paul Matthews blamed the turmoil surrounding Bear Stearns and the Bank of Japan for the troubles. He said the firm would remain in business.