Also found guilty were two of its contractors, Willmott Dixon Construction Ltd and PA Realisations Ltd (formerly Pectel Ltd), who breached safety regulations when removing asbestos-containing materials during the refurbishment of stores in Reading and Bournemouth..
The work was carried out on the stores between 2006 and 2007.
In the case, brought by the Health and Safety Executive (HSE), Winchester Crown Court heard how construction workers at the two stores removed asbestos-containing materials that were present in the ceiling tiles and elsewhere.
During the 15-week trial, the court heard that, as client, Marks & Spencer did not allocate sufficient time and space for the removal of the asbestos-containing materials at the Reading store. The contractors had to work overnight in enclosures on the shop floor, with the aim of completing small areas of asbestos removal before the shop opened to the public each day.
The HSE alleged that the retailer failed to ensure that work at Reading complied with the appropriate minimum standards set out in legislation and approved codes of practice. The company had produced its own guidance on how asbestos should be removed inside its stores, and the court heard that this guidance was followed by contractors inappropriately during major refurbishment.
The contractor, PA Realisations Ltd, failed to reduce to a minimum the spread of asbestos to the Reading shop floor. Witnesses said that areas cleaned by the company were re-contaminated by air moving through the void between the ceiling tiles and the floor above, and by poor standards of work.
The principal contractor at the Bournemouth store, Wilmott Dixon Construction Ltd, failed to plan, manage and monitor removal of asbestos-containing materials. It did not prevent the possibility of asbestos being disturbed by its workers in areas that had not been surveyed extensively.
According to the HSE, asbestos is the biggest single cause of work-related deaths in the UK, with an estimated 4,000 people dying every year.
The guilty companies will be sentenced on 26 September 2011.
After the hearing, HSE principal inspector Charles Gilby said: "This prosecution exposed serious failures by Marks & Spencer and its contractors that we hope others will learn from. This verdict is a wake-up call for the retail industry. Client accountability and responsibility is at the heart of this case, because asbestos can and does kill.
"There are very real lessons here for the country's large retailers and other organisations engaging in programmes of refurbishment, that they must allow enough time and resource to carry out work without endangering anyone."
Marks and Spencer plc, of Waterside House, North Wharf Road, Westminster, was found guilty of breaching section 2(1), relating to its own staff, and section 3(1), relating to members of the public and other workers, of the Health and Safety at Work etc Act 1974. These charges relate to the Broad Street Reading store and date from 24 April to 13 November 2006.
Willmott Dixon Construction Ltd, of Hertfordshire, was found guilty of contravening sections 2(1) and 3(1) of the Health and Safety at Work etc Act 1974 between 5 February 2007 and 28 February 2007. These breaches took place at the M&S store in Commercial Road, Bournemouth.
PA Realisations Ltd (formerly Pectel Ltd), of the Observatory, Chapel Walks, Manchester, was found guilty of contravening regulation 15 of the Control of Asbestos at Work Regulations 2002 between 5 May 2006 and 12 November 2006 at the M&S store on Broad Street, Reading.
At an earlier hearing, Altrincham-based Styles & Wood pleaded guilty to contravening sections 2(1) and 3(1) of the Health and Safety at Work etc Act 1974. These charges relate to offences committed between 24 April and 13 November 2006 at the M&S store on Broad Street, Reading.