Public tickets for the Manchester benefit concert have sold out within 20 minutes of going on general sale.
But some tickets have been reserved for fans who attended last Monday’s Ariana Grande show who have not yet been able to claim free places.
Those fans will now be able to register for tickets to One Love Manchester until 14:00 BST on Thursday.
Robbie Williams and Little Mix have also been added to the line-up for the concert at Emirates Old Trafford.
They join Grande, Take That, Niall Horan, Katy Perry, Miley Cyrus and Justin Bieber.
One Love Manchester takes place on Sunday, with proceeds going to the Red Cross’s Manchester Emergency Fund.
The event will come less than two weeks after 22 people died and 116 were injured in a suicide bombing after Grande’s concert at the city’s arena.
Ticketmaster said its website experienced “remarkable demand” when tickets went on sale at 10:00 BST on Thursday.
The company also said it was aware of some fans who were at the original show who had not yet been able to claim their places for this Sunday’s event.
Ticketmaster said: “We would like to reassure any fans who were at the [Ariana Grande] show that they will receive a free ticket.”
Ticket buyers who were at the concert at Manchester Arena on 22 May have been asked to provide their original booking details in order to register.
In a statement, Ticketmaster said: “We understand that there are a number of fans who didn’t receive an email this morning with regards to free tickets for One Love Manchester.
“We worked through the night and verified thousands of original bookings although there were some we could not verify.
“We want to give all fans who were at the show, regardless of where they bought their tickets, every opportunity to register for One Love Manchester, so we have opened up the registration again today, until 2pm.”
Some tickets for the benefit concert have appeared on eBay – but the company has told the BBC it is attempting to remove listings as soon as they appear.
Ticketmaster UK said it was “cancelling tickets of those who are reselling wherever possible“.
Some concertgoers who bought their tickets for Grande’s original show from secondary ticketing sites had initially been told they would not be eligible for free tickets to Sunday’s concert.
But those websites are now making efforts to ensure that tickets are offered to the people who actually went to the original gig, rather than those who first purchased the tickets.
Sunday’s concert, to be held at the Emirates Old Trafford cricket ground, will be broadcast on BBC One and is expected to raise more than £2m for the Red Cross fund.
The Black Eyed Peas, Coldplay, Usher and Pharrell are also going to perform.
Fans have been asked not to bring bags to avoid delays going through security.
ITV has made schedule changes to avoid the final of Britain’s Got Talent clashing with the benefit gig.
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