Jocelyn Stevens, the chairman of English Heritage, has apologised to a senior member of his staff who threatened to resign after a heated argument about the future of an important art deco building in west London.

The battle over the Smith-Kline Beecham factory in Brentford, featured in Independent London in May, also involves Richard Rogers, one of the country's leading architects, who has designed a replacement building.

Conservationists believe it is the most crucial issue since Lord Palumbo's successful attempt to replace listed buildings at No 1 Poultry site in the City with a building designed by the late Sir James Stirling.

Last week a row erupted between Mr Stevens and Paul Drury, head of the heritage watchdog's London region. Mr Drury said he was going to resign after what he deemed to be repeated and heated criticism of his department's work by Mr Stevens, who was unhappy about the handling of a planning application to demolish the building.

Independent London has obtained a copy of a letter to Hounslow Council, the planning authority, dated 4 July and signed by Dr Gordon Higgott, historic buildings advisor for the London Region. It outlines the position which led to the difference of opinion between Mr Stevens and Mr Drury. It states the demolition proposals 'are unacceptable and cannot be justified'. It 'formally directs (Hounslow) council to refuse (listed building) consent on this application'.

Reasons for the blunt advice include:

the building's intrinsic architectural and historic importance;

the fundamentally sound structure of the building which make refurbishment a viable option;

the plan is outline only and may never be realised, and,

the features said to benefit the local community are 'dubious'.

Insiders at English Heritage confirm that the row between Mr Drury and Mr Stevens broke out on the night of Tuesday, 19 July during a meeting with Mr Rogers. It has now emerged that Mr Stevens believed more time should have been spent liaising with those proposing demolition before the decision to refuse consent had been made.

Mr Drury felt he was forced to defend his staff's letter detailing the reasons why the council should refuse consent for demolition for the former aircraft component factory, built in 1937 on the Great West Road. The London region had been against this scheme as early as January.

A similar dispute erupted the following morning at a meeting of English Heritage Commissioners. At this point Mr Drury threatened to resign, agreeing to work out his notice and leave in mid-October.

Two days later, Mr Stevens, in an unusual step, issued his public apology. In a letter to the editor of the Independent, Andreas Whittam Smith, Mr Stevens said Mr Drury had not resigned, and denied launching an attack on English Heritage's decision to oppose demolition of the building.

'I did question whether more time should have been spent with the applicants before a decision which could lead to a public inquiry had been taken,' he wrote.

'I acknowledge that I did so in a forceful manner and I have subsequently apologised to Mr Drury for expressing my concern about the procedures used in such cases in terms which appeared to be a criticism of him personally and the staff of our London Region rather than the planning system.'

Meanwhile, the local row over the plans has been heightened by the one at English Heritage.

According to opponents, the demolition plans fly in the face of all statements by Hounslow council that it was committed to preserving buildings on the Great West Road which made a specific contribution to the historic and architectural character of the area. (SmithKline Beecham financed the report into the future development of the Great West Road, in which this strategy was outlined.)

In its attempt to persuade the council of the need to knock down the Thirties headquarters, SmithKline Beecham commissioned a report which concluded that it would not be cost-effective to retain the building and redevelop it either as top quality or even a basic office building. But a study for English Heritage, by leading surveyor Drivers Jonas, concluded that providing the area around the headquarters was developed, it would be possible to refurbish the building to be economically viable. This formed part of the response contained in the controversial letter to the council from English Heritage, which has been obtained by Independent London.

Heritage officials also refute the claims by the SmithKline Beecham report that the building is of minor importance because it is outside the mainstream of the Thirties modern movement. They say it is a fine and well-preserved example of a monumental art deco factory.

English Heritage also questioned the viability of such a huge commercial venture as the one planned. 'The advantages to Hounslow from the redevelopment . . . are far from guaranteed and. . .the demolition of the listed building cannot be justified on the basis of these promised advantages.

A new park, which was part of the development, would be in the shadow of the new building most of the day which, combined with pollution and noise from the nearby M4, would 'render a significant portion of the proposed park uninviting and therefore little used.

The report also raises questions about the planning application. The proposals are in outline only so, by approving them, the council would have no guarantee that what is proposed at this stage would be built. If SmithKline Beecham gets the go-ahead to demolish the building, thus raising the potential of the land on which it stands and the surrounding area which is owned by British Land, it could still change its final plans.

The row over the site leaves a question mark over what will happen next. Legally, English Heritage cannot backtrack on its stance, despite the reported objections of Mr Stevens.

SmithKline Beecham could submit a modified planning application, and English Heritage has made it clear it would consider an alternative plan, providing it retains the art deco building. Unless the matter is resolved, either English Heritage or SmithKline Beecham could call on the Government to hold a public inquiry into the demolition plans.

Joseph Mirwitch, a local spokesman for the 20th Century Society, which opposes the plans, believes the council was initially attracted to the scheme by the promise of other benefits for the area, financed by the drug company.

As part of the application to demolish the headquarters, SmithKline Beecham spent pounds 1,000 on promoting its plans for a new office block which would 'bring it into the 21st century. It also agreed to finance improved public transport links to the site, including a new British Rail station between Kew Bridge and Brentford, linking directly to Waterloo, and to build a park around the new office block.

'It would be an appalling act of vandalism if this building of national and international importance were arbitrarily destroyed,' said Mr Mirwitch.

The Beechams factory, as it is known locally, was built for Simmonds Aerocessories as an aircraft parts manufacturer. After the Second World War, it was bought by BOAC, who converted it into offices then sold it to Beechams in 1955.