Royal & Sunalliance posts a 9% rise in operating profit to £502m worldwide in the nine months to September 30.
General insurance premiums rose by 19% to £6.33bn, primarily due to the acquisitions of Orion in the US and Trygg-Hansa in Scandinavia.
But excluding these two deals, net written premiums edged up just 1%.
Bob Mendlesohn, chief executive, said there had been considerable rate rises in the UK.
"Both commercial and personal motor have seen substantial improvement, with the scale of that improvement being masked by the additional costs arising from the recent UK law reform concerning awards for pain and suffering.
"The claims management initiatives that have been introduced in the UK have now achieved savings of £30m across the household and motor accounts," he said.
Mendelsohn also allayed fears that the insurer would suffer serious losses from the recent flooding which he said would cost the group between £80m and £110m.
He said he wanted to dispel concern that losses would approach those incurred during the 1987 hurricane in the UK, which had cost the group's predecessor companies £450m.
He added: "The strong and accelerating rate increase across the commercial lines which we reported at the half year have continued.
"Year-to-date increases in these lines of business in the US are averaging more than 15% and running at 12% in Australia."