It will not charge donors or charities a subscription or sign-up fee, or take a commission. However, it will not pay the fees charged by banks for debit and credit card payments, which will be deducted from donations.
According to a spokesman for BT, a donation of £10, for example, would have 15p deducted from it if it was made by debit card and 13p if it was made by credit card. These charges would go to the credit and debit card companies.
Charities will be allowed to receive Gift Aid on donations made through the site.
The company said it would run the site for free as part of its charitable giving programme, and has also committed to investing a minimum of 1 per cent of its pre-tax profits in community and environmental programmes.
BT said the site was open to all UK-registered charities and would allow fundraisers to build and customise their own events pages. It will also enable them to link to social media sites, such as Facebook and Twitter, in order to promote fundraising events.
Ian Livingston, chief executive of BT, said: "We've launched MyDonate because we want to ensure that 100 per cent of people's fantastic fundraising efforts and donations go to the charities of their choice."
Nick Hurd, the Minister for Civil Society, said this was a great example of how business could play a part in building the big society.
Andrew Flanagan, chief executive of the NSPCC, said he thought the site would encourage more people to give online.
The site has opened to fundraisers who wish to raise money for charities registered on it. Charities that want to sign up to the site should visit the MyDonate website.