Tickets agencies are struggling to cope with the "sheer volume" of Take That fans trying to buy tickets for their tour.
Fans complained about websites crashing and phonelines being constantly engaged after the tickets went on sale at 9am on Friday.
A spokeswoman for Ticketmaster said: "Due to totally unprecedented demand all ticket agents across the UK, including Ticketmaster, are struggling to cope with the sheer volume of people logging on to try and buy Take That tickets.
"The knock-on effect is also creating problems for the UK telephone network. It's been confirmed that it's also straining to cope and that call volume is at least twice that of the Michael Jackson on sale last March. We are working to resolve the issues and we are confident that fans will be able to purchase tickets and we ask that they keep trying."
Fans logged on to social networking sites to complain about the situation.
One fan, Laura Docherty, wrote on the band's Facebook page: "I got tickets. But I've never experienced every website down. Got tickets in the end. Good luck to you all who are trying."
The band, newly reunited with fifth member Robbie Williams, announced details of the long-awaited tour on Tuesday and promised a "feast for the imagination".
The summer stadium tour around the UK, which is their first tour as a complete group in 16 years, includes four nights at Wembley Stadium after it kicks off next May. The last time Williams, Gary Barlow, Mark Owen, Jason Orange and Howard Donald performed together was in 1995.
This 14-date Progress Live 2011 tour will end with six dates in Europe and feature Take That's biggest hits from the last two decades.
The band later announced they will perform an extra gig at Sunderland's Stadium of Light on both Monday 30 and Tuesday 31 May, 2011, after the unprecedented demand for tickets. Extra dates were also announced in Glasgow, at Hampden Park, on Friday June 24 and in Birmingham, at Villa Park, on Tuesday June 28.