The slogan has been challenged by an employee of Friends of the Earth, who ran a website which listed environmentally friendly certified printers.

The report said the company sent the mailer to "fewer than 475 businesses" and said it was designed in good faith.

BSP said it had confirmed with the BPIF that no other members had achieved equivalent accreditations: it is ISO 14001, FSC and PEFC accredited.

It said all its printing inks were vegetable-based, and 100% of the energy it used came from renewable sources.

BSP managing director Simon Hunt, formerly of Goodman Baylis printers in Worcester, said: "The mailer was sent out as a gentle, tongue-in-cheek reminder to 275 of our customers.

"It was sent out in late November at the end of last year. We can't back up that we are the UK's leading eco-friendly printer, but we think we are one of environmentally friendliest."

BSP also said it would be imminently accredited as the first BPIF/British Standards Institute-accredited carbon neutral printer.

However, the ASA said it understood from the BPIF that the company did not keep a comprehensive record of the different environmental accreditations.

It therefore has not provided confirmation that BSP was the leader in that respect. It also said that several printing companies had achieved the same accreditations, and that the company had not yet been classified carbon neutral.

The ASA is the independent body set up by the advertising industry to police the rules laid down in the advertising codes.