Global firm Baker & McKenzie is expanding its London office and is set to take over the whole building it currently shares with investment bank JP Morgan.
Baker & McKenzie will become the sole tenant at 100 New Bridge Street under an agreement with its landlord, British Land.
Russell Lewin, Baker & McKenzie managing partner, says: "Our expansion over the past couple of years, coupled with our plans for future growth, have made obtaining additional space an urgent requirement."
Baker & McKenzie took 80,000 square feet of space when it first occupied its City premises.
The Baker & McKenzie block has a panoramic view of St Paul's Cathedral from the upper storeys.
But the cramped conditions have forced the firm to negotiate for more space in the building.
A Baker & McKenzie spokeswoman says: "We are absolutely jam-packed."
The extra space will enable the firm to bring its London presence together under one roof.
It is to be reunited with its property department, which comprises three partners and 20 staff.
At the moment the property department is in Fleet Place, around the corner from the firm's headquarters.
JP Morgan is to give up half its space over the summer and the bank will have pulled out completely by 2001.
Two weeks ago Baker & McKenzie was named Best Global Law Firm at The Lawyer Awards.