Online sales were most impacted, falling 11%, while the total amounts wagered in the UK declined 2%.
The figures follow a profit warning in March, when the UK’s largest bookie said dire horse racing results at the Cheltenham festival and high-value internet gamblers imposing restrictions on their own betting would lower full-year trading.
William Hill reiterated its expectation that operating profit for 2016 will fall to between £260 million and £280 million, compared to £291 million last year.
Chief executive James Henderson said: “It has been a tough start to the year in online, which is being impacted by both regulatory change and a gross win margin below normalised levels for the period due to a disappointing Cheltenham festival and unfavourable European football results.”
Last year the company was hit by almost £90 million of new online and gaming machine levies.
However, the firm flagged an improvement in its retail gaming machine arm, which grew 9%, and positive trends from its Australian operation, with wagers up 22%.
The firm’s annual general meeting will also see a vote on directors’ pay, which comes amid rising shareholder concern over high executive payouts that bear little correlation with company performance.
However, Mr Henderson’s pay last year was £914,417, a 35% fall from the year before.
The firm has also committed itself to paying the national living wage of £7.20 an hour to all 12,500 staff at its UK shops over the age of 18, seven years below the legislation being introduced by the Government later this year.
The company estimates that 4,500 staff will benefit from the pay rise, including 2,000 staff between 18 and 25.