BT has put together an online calculator, which works out how much CO2 a business produces annually by looking at energy consumption and work-related travel.

The website also contains hints and tips on how small enterprises can become more environmentally friendly.

A BT spokesman said:

"SMEs can take real steps to reduce the size of their carbon footprint and collectively have global impact.

"They can use this as a powerful differentiator with customers who want to see businesses take leadership on this issue. If you run a small business, visit our site and see for yourself the difference you can make."