Wilde Sapte is bound by an agreement to lease one of its floors to accountancy giant Deloitte & Touche, despite needing the space following its merger with Denton Hall.
The sublet contract was signed in September, two months after Wilde Sapte consented to lease accountants the eighth floor of its state-of-the-art offices at 1 Fleet Place, London.
The agreement runs from the start of October.
Steve Blakeley, Wilde Sapte managing partner, says: "We didn't know we were going to merge then. We are talking about 18,000 sq ft. We now have no spare space. Having done the merger we need that.
"There were people working in this office space. We were spread out over several floors. We are relocating these people into the other part of the building.
"If we had known in July that we would succeed in a merger with Denton Hall by the end of September we would not have sublet it because we would be confident of growing into the space we have got.
"The idea was cutting overheads. It was office space that we were occupying but we were doing it in an inefficient manner.
"When we moved here the idea was that we would grow so we should fill it.
"When we looked to merge with Arthur Andersen we thought we would fill it. We did not sublet it while we reviewed our options."
It is understood the office space will provide a home for 100 or so staff from Deloitte & Touche's insolvency team.
Wilde Sapte began negotiations with Denton Hall in September and a merger was pushed through over five days last month (The Lawyer, 18 October 1999).
The merger begins in February 2000. The new firm will take the name Denton Wilde Sapte and will have 200 partners, nearly 800 other legal staff and more than 1,500 staff in total. Wilde Sapte currently has 290 fee earners and 62 partners.
Denton Hall has 138 partners, 360 legal staff and more than 1,000 staff worldwide.