Organisers have so far confirmed 17 acts for Wembley

An eBay spokesman said the concert was a special event and it made sense for good causes to benefit from resales.

Live Earth said 203,000 people had registered for 60,000 tickets for the 7 July concert at Wembley.

The �55 tickets, at prices of up to �300 each, began to appear on eBay soon after they had been allocated.

Successful fans have until 1200 BST Friday to claim them.

Good causes

Madonna, James Blunt and the Red Hot Chili Peppers are among the 17 acts confirmed for the concert, which supports charities fighting climate change.

In the US, The Police, Kanye West and Bon Jovi will be among the artists at the Giants Stadium in New Jersey on the same date.

CONFIRMED WEMBLEY ACTS Beastie Boys Black Eyed Peas Bloc Party Corinne Bailey Rae Damien Rice David Gray Duran Duran Foo Fighters Genesis James Blunt John Legend Keane Madonna Paolo Nutini Razorlight Red Hot Chili Peppers Snow Patrol

The eBay spokesman said British sellers would be strongly encouraged to donate the money to Stop Climate Chaos, the organisation supporting the Live Earth concert in the UK, or to another charity supporting the climate change agenda.

He added: "Although the charity will have benefited from the original sale of the ticket, we think it makes sense to use our charity fundraising programme to ensure that good causes benefit from the resale of any spare tickets on the site.

"As a result, users who decide to resell their tickets will be required to donate at least 20% of the final sale price to good causes through our charity fundraising platform."

Marianne Troup of Live Earth UK said: "This is an independent decision by eBay and we are not involved with it.

"While we acknowledge the charity donation we are not comfortable with people making money from what is a social issue."

Global plans

In 2005 eBay banned Live 8 tickets, which were also allocated after text applications, after organiser Bob Geldof had branded the attempted sale of the free tickets as "sick profiteering".

Seven concerts are due to take place in major cities around the world over a single 24-hour period.

There are no plans to extend the charity charge arrangement globally.

The London show will be broadcast live on BBC television and radio and shown in 120 countries.