Standard & Poor's on Tuesday said it had lowered its credit rating for British oil and gas explorer Tullow Oil after it announced a review of its African business which would increase its exposure to Ghana.

The recent sharp declines in oil prices put further pressure on Tullow's profits outlook, S&P said, after it cut its long-term corporate credit rating for Tullow to "BB-" from "BB".

"Tullow Oil has recently announced potential adjustment in its investment plans, which could increase Tullow's exposure to Ghana. We consider that this represents a rating constraint," the rating agency said in a statement.