Linklaters & Paines is advising the Woolwich Building Society on its plans for conversion to plc status and London Stock Exchange listing.
Woolwich is the UK's third-biggest building society with assets of about £28 billion, 12 operating subsidiaries, over three million investors and more than 550,000 borrowers.
Heading Linklaters' team are partners Jeremy Skinner and Tom Wethered, with assistance from Andrew Peck. Peck is already advising biggest society Halifax in its merger with Leeds Permanent, which went through on 1 August 1995, and the new society's conversion due early next year. None of the partners could comment.
Woolwich said the greater freedom permitted by the law governing banks will give it "wider operational flexibility" to develop a bigger range of products and services, and more freedom to access the wholesale money markets and acquire and fund its assets.
Legal work to prepare for the move is also being done by the nine-lawyer, in-house team headed by Sue Scott.
The long-term treasury work undertaken by Linklaters for Woolwich helped the firm secure the job, said Scott.
Slaughter and May commercial partner John Macaskill said incorporation is a slow process due to the degree of scrutiny required by the Building Societies Commission and Bank of England before a banking licence is issued.
Macaskill, together with partner Tim Clark, is known for handling some of the top pieces of society conversion and merger business. Slaughters acts for N&P in its merger with Abbey National, due to be completed late this summer, and advised Cheltenham & Gloucester on its takeover by Lloyd's Bank for £1.8 billion.
The market for advising large societies on demutualisation is limited by the size of the sector. But Macaskill said lawyers with such experience recognise they will be in demand among the growing numbers of insurance companies seeking demutualisation.
"I'm quite certain all of our competitors are marketing themselves like mad," he said.
Alliance & Leicester, with around £22 billion assets, also confirmed it is "seriously considering" a similar plc move.
A source said legal advice was being sought and the board may finally decide whether to go ahead in the next two months.
The society would not say which law firm it is using.