Ticket registration for the UK Live Earth concert has officially opened.
The Red Hot Chili Peppers will be among the stars in London
The show at London's Wembley Stadium on 7 July will be one of seven worldwide concerts to highlight climate change.
All fans who want tickets must register online before 1200 BST on Monday. The £55 tickets - limited to two per person - will then be allocated by ballot.
Madonna and the Red Hot Chili Peppers will be among the stars performing. A spokesman said they had "no firm idea" about how much demand there would be.
"With this sort of line-up and the nature of the event we expect a lot of demand, but it's difficult to be sure," he said.
Unlike the Glastonbury Festival's registration system, concertgoers will not be asked for a photograph but must provide a contact phone number.
Other acts confirmed for the UK show include Snow Patrol, Razorlight, Corinne Bailey Rae, the Foo Fighters and James Blunt.
Around the globe, other Live Earth concerts will take place in New Jersey, Sydney, Tokyo, Rio de Janeiro, Johannesburg and Shanghai.
The Police, Kanye West and Bon Jovi will headline the US leg. More acts for all venues are still to be announced.
The concerts have been organised by former US vice-president Al Gore as a way of highlighting climate change.
All seven concerts are due to take place over a single 24-hour period, with profits going to organisations including the Alliance for Climate Protection, the Climate Group and Stop Climate Chaos.
The BBC has confirmed that it will broadcast the concerts on TV and radio in the UK.