April 12 (Reuters) - The Grand National was Britain's biggest ever online sports betting event as shop closures meant punters had to go online or use mobile apps to bet on one of the annual highlights of the horseracing calendar.

Entain, the owner of Ladbrokes, and rival William Hill separately told Reuters on Monday that the total takings were however lower than in previous years due to COVID-19 lockdowns.

Entain, which also owns the Coral brand, said more than 150,000 customers visited its websites every minute during the event, which took place on Saturday in front of empty stands after a pandemic-led cancellation in 2020.

Rachael Blackmore won at odds of 11-1, becoming the first female jockey to triumph in the race, which began in 1839.

"This year's Grand National event saw record-breaking betting volumes for our UK brands and huge customer excitement in line with the thrilling outcome," Dominic Grounsell, who heads Sports Brands at Entain, said on Monday.

Neither company can give specific figures but over 100 million pounds ($137.43 million) is estimated to have been bet on the race overall.

The event typically draws in bets from around 13 million people, roughly a third of adults in the UK, each year.

A compelling storyline should encourage some of those occasional gamblers to take a closer interest in the sport.

"The problem is that if we have a bad result we don't see the money coming back as for many it is their one bet of the year. This year was good and we did well considering we saw the first ever female winner," a William Hill spokesman said.

The expertise in online gambling and sports betting that the bookmakers have in Britain, which is amongst the first major countries to regulate the market, has attracted American suitors as the United States opens up to sports betting.

($1 = 0.7276 pounds) (Reporting by Muvija M in Bengaluru; Editing by Keith Weir and Ken Ferris)