Zurich is expected to increase its share of the solicitors' professional indemnity (PI) market during this year's renewals, according to PI brokers.
A leading PI broker said: "Zurich has probably widened the gap between itself and its rival insurers, principally St Paul.
"Zurich has been aggressive on the larger accounts and has won a lot of new business."
Zurich said it was "too early" to confirm whether it had increased its market share, but a spokeswoman said the insurer was "insuring more firms this year than last".
The spokeswoman added that Zurich had quoted for around 6,000 firms this year.
A St Paul spokesman said: "Until the results of the share of the premium are published, it's not possible to comment."
"We believe our consistent approach is good for law firms and good for the market. We are the Law Society's joint venture partner and we're here to stay."
Alexander Forbes Professions director Trevor Moss said solicitors had typically seen premium increases of around 10%-15% on the compulsory £1m of cover.
But he added that firms involved in residential conveyancing or niche firms involved in North American-domiciled intellectual property work may have seen increases of around 20%-30%.
Moss said premium increases for top-up cover had been in the region of 25%-30%.
But some firms have seen the price of their PI cover double.
"There are also a number of firms that have come out of long term agreements with both primary level and top up level underwriters and such firms may have experienced increases of up to 100%," said Moss.
It is estimated that hundreds of firms are yet to secure cover. "It's causing concern," said Moss.