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.
Ticketmaster said: "We would like to reassure any fans who were at the show that they will receive a free ticket."
Ticket buyers who were at the Ariana Grande concert at Manchester Arena on 22 May have been asked to provide their original booking details in order to register.
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.
One Love Manchester takes place on Sunday, with proceeds going to the Red Cross's Manchester Emergency Fund.
Ticketmaster said their website experienced "remarkable demand" when tickets went on sale at 10:00 BST on Thursday.
Last Monday, 22 people died and 116 were injured in a suicide bombing after Grande's concert at the city's arena.
The ticket company said it was aware of some fans who were at that show who had not yet been able to claim their places for this Sunday's event.
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."
- BGT final moved to avoid Grande clash
- How is a major concert organised at short notice?
- Fans' concerns over Ariana Grande tickets
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.
You can also contact us in the following ways:
- Tweet: @BBC_HaveYourSay
- WhatsApp: +447555 173285
- Text an SMS or MMS to 61124 (UK)
- Please read our terms & conditions
Or please use the form below: