Two of the UK's leading barristers have been called in to advise on the Northern Rock saga as the debate over City firm conflicts gathers pace.

Brick Court Chambers' Charles Hollander QC, one of the country's pre-eminent conflicts specialists, was instructed by Freshfields Bruckhaus Deringer to advise on the potential conflicting nature of the two roles the firm has subsequently accepted – for the Bank of England and then Northern Rock.

Jonathan Sumption QC, also from Brick Court, has been brought in by Clifford Chance to advise the Bank of England, although his role remains unclear.

The news comes as it emerges Allen & Overy (A&O) has also taken a second role in the process. The firm is advising Northern Rock on its rescue package with the Bank of England while also advising on Virgin Group's bid.

The decision by both firms to accept two roles has provoked fierce debate among City lawyers, especially at US firms, some of which claim to have turned away roles because of conflicts. One corporate partner at a US firm said: "This is a UK-firm problem. Under our conflicts rules we would have difficulty accepting those instructions even if they were for unrelated transactions."

A&O and Freshfields have vigorously defended their position saying they went through appropriate measures and obtained client consent where needed. Both firms also argue the transactions are separate, although A&O admits it has information barriers in place.

Freshfields and A&O have both previously been subject to Law Society conflict investigations respectively for their roles on bids for Marks & Spencer (2004) and Safeway (2003).