(Reuters) - Bank regulators forced JPMorgan Chase & Co to recalculate capital levels for its primary banking unit, resulting in lower readings of its cushion against losses, the company said on Thursday.

The revisions reduced the bank's Basel I Tier 1 common ratio to 9.9 percent from 10.3 percent at the end of June, JPMorgan said in a filing with the U.S. Securities and Exchange Commission.

The revisions came after the bank was informed on Wednesday by the U.S. Office of Comptroller of the Currency and the Federal Reserve Bank of New York that changes were needed. The changes reflect "regulatory guidance regarding a limited number of market risk models" used for positions in the first half of the year, including some at the company's Chief Investment Office, the bank said.

The Chief Investment Office is the site of the bank's nearly $6 billion of losses from the so-called London Whale derivatives trades.

(Reporting by David Henry in New York; Editing by Gerald E. McCormick)