In the late '80s the interior of the building, described by Pevsner as Finsbury Circus' 'one masterpiece', was 'rebuilt with tact and imagination' to a design by Inskip & Jenkins.
Gaunt Francis partner, Alan Francis, said: 'Inskip & Jenkins created a new atrium with five lifts and a linking balcony. They also removed most of Lutyens' office floor plates.
'We are taking away those five lifts and the balcony and putting on a new atrium roof that will allow more light into the building.
'We will also upgrade the original Lutyens reception to create a 21st-century one. Everything that one 'touches' - IT controls, lift buttons, security controls - will be more contemporary, but one will still be able to recognise Lutyens' original work,' added Francis.
'Done with lavish expense -in a style that owes much to North Italian Mannerism,' Lutyens started work on the building for the Anglo Persian Oil Company (later BP) in 1921 and it was completed in 1925.
The new scheme, which will create more than 16,000m2 of office space, is expected to be completed in February 2008.