Conservation watchdog English Heritage has released details of a report written a year ago which advised the Government that the NatWest Tower in Birmingham city centre should not be listed as a building of special architectural importance.

A study into the Colmore Row landmark concluded that the brutalist 1970s structure had been considerably altered over the years and lacked the “high degree of architectural quality and sophisticated detailing” necessary for listing to take place.

English Heritage is in the process of re-examining its decision following a request by British Land, owners of the NatWest Tower, to demolish the building and construct a £160 million 35-storey skyscraper in its place.

British Land is seeking a certificate of immunity from listing from the Department for Culture, Media and Sport which, if granted, would give the company a five-year breathing space to demolish the tower.

An English Heritage spokeswoman said issues from the 2007 report on the merits of the NatWest Tower remained “very relevant”.

She added: “Slightly deeper questions are asked in order to grant a certificate of immunity, so we can’t be seen to pre-judge the outcome of our research.”

English Heritage’s 2007 study refers to an article in the Architectural Review in January 1969, which described the NatWest Tower as an “elegant steel and glass rescue operation in a frontage badly scarred by third-rate architecture”.

Original features remaining in the building, which has been closed for several years, include a private staircase to the manager’s office, which has panelled walls and is equipped with a drinks cabinet.

The report adds: “The banking hall is the most interesting of the interiors of the building and is well preserved with the great majority of its original fittings intact, including the overall plan form and details such as light fittings, switches, door furniture, decorative plaster panels and tile work to the walls and the coffered ceiling. The component parts of the building are well grouped and form an effective addition to the streetscape of this part of central Birmingham and to the skyline.”

The English Heritage report sheds light on a lively discussion in the architectural world about the work of John Madin, who designed the NatWest Tower and Birmingham Central Library, another brutalist building threatened with demolition.

According to the Pevsner Architectural Guide for Birmingham by Andy Foster, Mr Madin’s practice “consistently produced the city’s best architecture” from 1962 to 1992.

However, expert analysis provided for English Heritage by British Land describes the Madin Partnership as an essentially commercial practice “whose lack of prestige was attested by their failure to win any large-scale commissions on university sites, especially in Birmingham”.