Fitch Ratings has lowered its issuer default and outstanding debt ratings for Burger King Holdings Inc., saying the company's debt is likely to increase once the fast-food chain is sold to a private equity firm.

Investment firm 3G Capital has agreed to buy Burger King for $3.26 billion and assume $753.7 million of secured bank debt. Burger King's board has unanimously approved the offer.

Fitch said there's a likelihood that debt will increase considerably to finance the deal.

The ratings firm cut Burger King's issuer default rating to "B+" from "BB," and affirmed its rating on the company's secured bank facility at "BB+."

Fitch also placed Burger King's issuer default rating on rating watch "negative," and assigned a recovery rating of "RR1" to the secured bank debt.

The firm said it believes the vast majority of the $4 billion deal value will be debt-financed.

Shares in Burger King Holdings Inc. fell 4 cents to $23.55 in aftermarket trading after adding $4.73, or 25 percent, to $23.59 during the regular session.