The site allocating tickets for the London 2012 Anniversary Games is experiencing delays.
It comes after a similar system was criticised during the Olympics last summer, which meant people missed out on seeing the events that they wanted to.
However if you have already registered the 'Ticket Master' links and website are still working.
To try and get tickets, you must register. You can do this at British Athletics website, although it is experiencing problems at the moment.
On Sunday, Paralympic heroes Jonnie Peacock, Richard Whitehead, Mickey Bushell and Josie Pearson will take on some of the best athletes in the world in a one-day IPC International Challenge, fresh from competing at the IPC World Championships in Lyon.
Paralympic T44 100m champion, Jonnie Peacock said: "There’s a huge positive energy in the Stadium and it’s a bonus that it’s a super quick track. It holds great memories for me and it will be great to go back there nearly a year on and see what I’ve got to offer.”
All five British Olympic and 14 of the 16 Paralympic medallists have, so far, been confirmed to compete this summer at the Sainsbury’s Anniversary Games.
In the IAAF Diamond League, Jessica Ennis will double up in both the 100m hurdles and long jump, before being joined in Saturday afternoon’s event programme by double Olympic champion Mo Farah, who will run in the 3,000m, and Olympic long jump champion Greg Rutherford.
Click here to register.
Enfield-born Team GB high-jumper Robbie Grabarz will be taking part in the Anniversary Games.
British Athletics' Ed Warner says Oscar Pistorius is unlikely to be invited to the Anniversary Games.
Warner says it is nothing to do with guilt or innocence, but to avoid a "media circus".
Sainsbury's has been named as the sponsor of the British Anniversary Games in the Olympic Stadium in July.
The three-day event includes the British Athletics London Grand Prix and a one-off Paralympic event.
Tickets go on sale on Friday.
British Athletics have announced that tickets for the London Anniversary Games, which takes place at The Stadium, Queen Elizabeth Olympic Park on 26th – 28th July, will go on sale exclusively to pre-registered fans at 10am on Friday 19th April 2013.
Pre-registration closes at midnight on 17th April and tickets will go on general public sale at 10am on 23rd April.
The majority of the tickets for the London Anniversary Games, which incorporates two days of IAAF Diamond League action and a one-off IPC International Challenge, will be priced between £12-£50.
Fans can register their interest in tickets at www.loveathletics.uka.org.uk