"Unprecedented demand" for EuroMillions lottery tickets led to a temporary technical fault, preventing some people from buying tickets online, Camelot has said.
The organiser apologised to those affected after some players were unable to use their online accounts to enter Friday night's £121 million draw.
In a statement Camelot said: "Some players were unable to update their card details or add funds to their National Lottery online accounts for a very short period of time.
"This was due to an issue with the response times from the online banking systems, which meant some transactions were unable to be completed.
"Any players who already had funds in their account would have been unaffected. We would like to apologise for any inconvenience this may have caused."
British lottery hopefuls were left disappointed after the mega jackpot was scooped by two foreign ticket holders.
The two winners of the £121,722,017 prize came from Portugal and Belgium, the National Lottery said.
The big jackpot was accumulated due to the prize rolling over six times.
The night's winning EuroMillions numbers were 12, 21, 06, 33, 27 and the EuroMillions Lucky Star numbers were 07 and 03.
Since June 2009, the EuroMillions jackpot has been won 14 times by people from the UK including an anonymous £113 million winner in October last year. This was Britain's biggest ever lottery win.