Car insurance firm Admiral Group is expected to list roughly a one-third stake in an initial public offering that will raise £225m next month.

The offering will value the entire company at up to £680m and is expected to include an option to sell an additional 15%. The shares are expected to begin trading on September 22. Admiral declined to comment.

However, the firm released first-half figures, announcing a 30.5% increase from the same period last year in adjusted core profits to £45.3m. Its turnover rose 30.2% to £269.3m.

Admiral, which specialises in providing insurance to people who pay higher premiums, such as those with high-performance cars, wrote total premiums of £233m, up 30.3% from the same period last year. The results were helped by the success of its online service elephant.co.uk, growth in premiums at Diamond, which insures women, and an "excellent" underwriting performance.