BOSTON (Reuters) - Billionaire hedge fund manager William Ackman has apologized to clients for betting on Valeant Pharmaceuticals International Inc VRX.TO, telling them he was "deeply and profoundly sorry" for losing so much of their money on the investment.

William Ackman speaks during the Sohn Investment Conference in New York May 4, 2015. REUTERS/Brendan McDermid

Ackman decided to sell his entire Valeant position earlier this month, suffering a roughly $4 billion loss since having bought the stake in early 2015.. He called the investment a “huge mistake.”

“My approach to mistakes is that I personally assume 100 percent of the responsibility on behalf of the firm,” he wrote in the firm’s annual letter released to clients on Tuesday evening and seen by Reuters on Wednesday.

The 50-year-old manager acknowledged the toll the bad bet has taken on his image and said he misjudged the management team in place when he bought the stock.

“We deeply regret this mistake, which has cost all of us a tremendous amount,” he wrote.

Thanks largely to Valeant’s tumble, Ackman’s hedge fund Pershing Square Capital Management suffered back-to-back losses in 2015 and 2016 as his reputation as one of the hedge fund industry’s most talented investors dimmed.

Since launching the firm in 2004, Ackman has delivered a compound annual net return of 14.8 percent. He tends to take concentrated bets and often pushes management to perform better by urging spin-offs or other measures.

Nonetheless the board of Pershing Square Holdings, Ackman’s publicly traded investment vehicle, decided after a review of his performance that he should continue to manage the investments, Anne Farlow, the chair, wrote in a separate letter.

She welcomed Ackman’s openness in analyzing what led to the Valeant failure.

In his letter, Ackman laid some blame at the feet of Valeant's former management team, which he had thought was building the next Berkshire Hathaway BRKa.N, once of the most profitable companies in the country.

Ackman met former Valeant Chief Executive Michael Pearson in 2014, when Pearson enlisted Ackman’s help to try and buy Allergan and Pershing Square bought up Allergan shares to try and push that company’s management into selling to Valeant.

Allergan ended up selling to a Actavis, netting Ackman his best-ever returns with his Pershing Square LP fund gaining 36.9 percent in 2014.

Ackman fired Pearson in 2016, however, after he got a seat on Valeant’s board.

“Prior management substantially overpaid for the company’s largest acquisition - its acquisition of Salix - which occurred contemporaneously with the substantial majority of our investment in the company,” Ackman wrote.

Pershing Square Holdings has swung to losses of 2.5 percent after starting the year with gains, but Ackman promised a quick recovery in the letter to clients.