Ratings agencies have sounded a warning bell about the potentially high levels of debt involved in two of this year’s biggest public-to-private buyouts, just as the kind of deals which typified the industry's boom years are experiencing a revival.

Standard & Poor's has put UK car parts maker Tomkins on credit watch with negative implications a day after US buyout firm Onex and the Canada Pension Plan Investment agreed to acquire the company for £2.9bn (€3.5bn), in what would be the biggest take-private buyout deal since April.