In a highly controversial and unprecedented move, the judges of the £20,000 Stirling Prize, added Colin St John Wilson & Partners' British Library to the list of finalists, after they had been handed a shortlist selected by juries.

The red-brick building, which cost more than £500m and was completed almost 20 years after first being approved, brings the number of finalists to 11.

It is up for the overall Building of the Year prize despite not being among the 10 category prize-winners in the Royal Institute of British Architects awards (Riba) .

A spokesman for the Royal Institute of British Architects, Tony Chapman, said the rules of the competition allowed the judges to select the building for consideration.

"They have the right to include any additional building they see fit," he said.

"It is unusual because it's never happened before, but it's not unconstitutional because they do have the right."

Costly criticism

Sir Colin St John's project at St Pancras in London was dogged by criticism and construction problems from the outset.

The budget overran by £350m and the work dragged on five years longer than originally planned.

Space for 12 million books and 1200 readers

Its design was deplored by leading figures including Prince Charles who once said it looked like a secret police building.

The criticism cost Sir Colin dearly. After it was completed his architectural practice went in to receivership as he could get no work.

The judges' chairman, David Rock, president of Riba, said he was personally pleased to see the library added to the shortlist.

"As well as being a fine building, especially internally, it represents a monumental achievement in the face of adversity," Mr Rock said.

Public opinion is also taken into account in the final analysis. A shortlist exhibition is opening at the Riba Centre from Monday where votes can be cast.