The head of one of the UK's biggest cab companies, Addison Lee, faces possible prosecution for instructing his drivers to ignore the law and use bus lanes in London.

John Griffin, Addison Lee's chairman, wrote to the company's 4,000 drivers in April and formally instructed them to drive in the capital's bus lanes, which are restricted by law to buses, coaches, bicycles, motorcycles and London's licensed black cabs.

The Metropolitan police have written to an MP who asked whether Griffin's order constituted an offence to say that the matter had been passed to the Crown Prosecution Service for advice on a possible charge. The move does not mean Griffin will definitely be charged. However, it implies that police believe the possibility of a prosecution is realistic enough to seek further advice.

Griffin, who campaigned successfully for the removal of a similar lane on the M4 motorway near London, argued that allowing licensed cabs to use the lanes but not minicabs amounted to "unfair and discriminatory treatment". He promised to pay any fines incurred by his staff.

His instructions prompted an immediate backlash, with Transport for London (TfL), the authority responsible for travel in the city, warning that Addison Lee drivers could lose their minicab licences for repeated offences. Later that month TfL won a high court injunction preventing the company from ordering its drivers to use bus lanes. Addison Lee, which says its drivers received around 800 tickets for bus lane use, withdrew the instruction ahead of a judicial review next month.

TfL wrote to the Metropolitan police to ask about the law in relation to Griffin's instruction. A Labour MP, Ian Austin, who co-chairs the all-party parliamentary cycling group, also wrote to the police on the issue. Commander Adrian Hanstock from the Met's Safer Transport Command replied to Austin, saying: "Mr Griffin's letter (to his drivers) has been passed to the Crown Prosecution Service for consideration, and the MPS are awaiting the outcome of this legal advice. I will send you an update once that advice has been considered."

An Addison Lee spokesman said he was unaware of the move and that police had not interviewed Griffin.

Griffin's instruction to staff brought a particular outcry from cycling groups, who warned that allowing London's minicabs to use bus lanes would see them filled with vehicle traffic when, at the moment, they offer something of a refuge for cyclists. They were further angered by Griffin's column in that month's Addison Lee in-house magazine, which argued that deaths and serious injuries among inner-city cyclists were inevitable and primarily the fault of untrained riders.

The bad publicity saw several hundred cyclists stage a "die-in" outside the Addison Lee headquarters, while the Department for Work and Pensions opted not to renew a contract with the firm.

Austin said Griffin should never have issued the instruction to his drivers. He said: "John Griffin might think he is funny, but the police clearly take a different view and given the number of cyclists killed and injured, many people will think it is dangerous and irresponsible for someone in his position to encourage conflict on the roads and tell his drivers to ignore the rules."

The Addison Lee spokesman said Griffin's order had been motivated in part by delays in securing a date for the judicial review, which will look into the wider issue of bus lane usage, and that this was now scheduled for mid-June.

In the wake of the publicity, he said, the company had introduced new training for drivers to help them be more aware of cyclists.

It emerged in April that Griffin's firm donated £250,000 to the Conservative party over three years, and that he was granted a meeting with the then transport secretary, Philip Hammond. Griffin raised the bus lane issue with Hammond at their meeting in October, according to minutes released under the Freedom of Information Act.