NEW YORK, Aug 1 (Reuters) - Bond insurer Ambac Financial Group Inc (ABK.N: Quote, Profile, Research) said on Friday it reached an $850 million counterparty settlement over derivatives that it had insured, which will help boost a unit's capital position.

Ambac said it had already recorded about $1 billion in mark-to-market losses on the securities, which were once highly rated but have since been downgraded to junk status.

As a result of the transaction, the company will be able to record a $150 million mark-to-market adjustment and boost the capital position of Ambac Assurance Corp, its main business unit. (Reporting by Lilla Zuill, editing by Gerald E. McCormick)