Fit-out contractors are believed to have put work on hold until the dispute has been sorted out.
Grenville Barr, Abbey National's business development manager, said the completion date for the project was still unclear. It was originally due to have been handed over in April.
Barr said project specifications agreed at contract stage had not been attained, and that British Land, developer for the project, and Skanska were working to resolve the difficulties.
Barr said: "To the eye, [the project] is virtually finished, but problem areas still to be finished are the highly specified interrelationship of all engineering system, including things like air-conditioning and fire alarms."
A project insider confirmed that there was a dispute over the interpretation of the specifications.
It is understood that senior executives at British Land and Abbey National, including the chairmen of both firms, have held meeting to resolve the difficulties.
A British Land spokesman said: "The delays are the subject of discussions between British Land, Abbey National and the contractor."
Skanska signed the £65m contract to construct the seven-storey building in 1999. The work included the fit-out of the building with carpeting, partitions and IT facilities.
The firm declined to comment.
Architect on the project is Shepherd Robson, the QS is Gleeds, and the structural and services engineer is Arup.
Earlier this year, Skanska had problems with other projects and disclosed losses of £13m on UK joint venture contracts with Costain and Mowlem.
Skanska refused to identify the contracts involved but it is understood that they included the £124m A2/M2 road widening scheme for the Highways Agency.
Skanska president and chief executive Claes Bjork defended the firm's UK strategy. He said: "We have a terrific business in the UK and it is doing very well."
The group issued a profit warning last week.