The opening weekend of the Fifa World Cup caused havoc for several online betting sites as users were unable to place bets during peak times, according to research from internet performance company Keynote Systems. 

BetUK, Sporting Odds, Bet365 and Betfair all suffered problems with their sites.

BetUK was slow to load on the day the World Cup kicked off, and failed to load at all during certain points in the afternoon and on Saturday night.

Research group Nielsen//NetRatings said that more than one million people went online on Saturday to read match reports and place bets. 

Keynote Systems said that the poor website performances could have influenced where people placed their bets, offering an advantage to sites including William Hill, Skybet and Betfred which held up a lot better during the weekend.

"Sporting events like this present a massive market opportunity for the gambling sites, and competition is fierce," said Haran Sold, EMEA managing director at Keynote Systems.

"It is critical that they do not fall foul of increased demand or technical glitches which can rear their heads at busy periods."