NEW YORK (Reuters) - Fitch Ratings on Monday revised its rating outlook on Barclays Bank Plc to negative from stable after the British bank agreed to buy Dutch rival ABN AMRO for about $91 billion (67 billion euros) in stock.

A negative outlook indicates greater chance of a rating downgrade and reflects execution risks Barclays will face in its cross-border merger with ABN AMRO, Fitch said in a statement. Fitch now rates Barclays Bank Plc "AA-plus," the second-highest investment-grade rating.

Fitch also said it may raise ABN AMRO Bank's rating, now "AA-minus," the fourth-highest investment-grade rating. The action reflects ABN AMRO's likely benefits from a merger with higher-rated Barclays, Fitch said.

Barclays and ABN said a merger would result in $4.75 billion (3.5 billion euros) of annual synergy benefits, largely from cost cuts including 23,600 job reductions, or just over 10 percent of the combined work force. For details see.