Churchill achieved more than 50% growth in gross written premiums in the first quarter, compared to the same period last year.
The boom reflects the group's expansion since it acquired the general insurance business of Pearl in June last year and Prudential in November. It also includes business written by fellow group member NIG.
Group managing director John O'Roarke said retention rates were "extremely strong" from both businesses.
He said: "We are extremely encouraged by the early results."
The group's rates for its motor and household cover were about 6% higher in the first quarter of this year than the same period last year.
O'Roarke said: "There has been strong growth from direct sales, the broker channel and partnerships.
"Part of it has also come from rate increases, but the contribution from rate increases is slowing."
Motor rates had reached a plateau, he said. But O'Roarke noted two problems ahead.
These were the continuing upward trend in personal injury claims and disappointing investment returns.
"Investment returns look set to be depressed for some time to come," he said.
Churchill pursues a relatively conservative investment strategy and keeps about 15% of its assets in equities leading it to have suffered less from last year's stock market losses than others.
Poor investment returns dragged Churchill's parent, Winterthur Insurance, into a net operating loss of SFr147m (£62.9m) in the first quarter, it reported.