The surprise inclusion is 17-partner firm Winckworth Sherwood, which is yet to develop a strong reputation in the field.
But a spokeswoman for St Paul says that it has confidence in the firm's litigation department, which includes Peter Williams, formerly of Capsticks, and Simon Eastwood, formerly of Hempsons.
The spokeswoman for the insurer says the firms have been chosen "because of their level of experience and because we need a good geographical spread".
The u21bn Minnesota-based insurer has also been hiring top London lawyers in-house to challenge the UK's mutual insurers.
It has poached Paul Yates, who is a partner at medical negligence specialist Hempsons, and Greg Waldron from Bolt Burdon in London, as well as newly-qualified Helen Kaney.
The trio are all dual-qualified as solicitors and medics. Kaney and Waldron are qualified dentists, while Yates is a doctor.
Yates says he has left Hempsons to get in "at the start of something that could really take off".
The medicolegal insurance market has previously been dominated by a number of mutual organisations, including the Medical Protection Society, the Medical & Dental Defence Union of Scotland and the Medical Defence Union.
But St Paul's move signals the arrival of private providers offering cheaper rates.
"With medical negligence claims being reported as increasing by 15 per cent a year we really felt that it was time doctors and dentists had a fresh option," says St Paul's international spokesman Dr Paul Lambden.
John Lamb, marketing manager for the Medical Protection Society, says private companies may look cheaper, but mutual and private insurers offer quite different packages that are not easily compared.
"We have seen attacks by commercial insurers in many countries that we operate in and they have got some business, but certainly the Medical Protection Society has not been particularly affected," says Lamb.
"They have not taken any significant share of the markets. Even in the US the large mutuals still have the majority share."