A Saga spokesperson confirmed work was "imminent" but said the firm had yet to make a decision on the nature of the repair work. He said: "We have yet to reach a conclusion. Recladding is the preferred option."
Problems with the leaks emerged soon after it opened in 1999. They became so serious that water was falling on desks.
Building disclosed last year that repairs were required. Other problems included the cladding system, which was designed to allow natural ventilation. This seems to have lead to draught problems.
The team working on the remediation work include some original team members, such as QS Davis Langdon, construction manager Schal and architect Bickerdike Allen.
A source said the programme of works would be carried out while the building was occupied. He said the cost of the works would partly be met by insurance cover, and that the rest was subject to legal discussions.
The team aims to retain the building's original design as much as possible. The source said that Saga chairman Roger De Haan was behind the decision to go ahead with the work. The source said: "It's been a bit of a cause célèbre. He want the problems sorted out."