LONDON (Reuters) - The number of Britons reporting they are employed on "zero-hours" contracts, which offer no guarantees on working hours, has risen by 20 percent year-on-year, official figures showed on Thursday.

The Office for National Statistics said it estimated 903,000 people, or 2.9 percent of Britain's workforce, were employed under zero-hours contracts in the second quarter of 2016, compared to 747,000, or 2.4 percent, a year earlier.

Employers using zero-hours contracts have come under fire for not giving their workers enough job security although the contracts are popular with students and some workers who want flexible hours.

Retailer Sports Direct said on Wednesday it would offer directly employed shop workers the option of switching from zero-hours contracts to ones with a guaranteed minimum amount of work as it tried to counter criticism of its treatment of staff.

The Trades Union Congress, Britain's largest union group, said its research showed a worker on a zero-hours contract earns 7.25 pounds per hour, as a median, compared with 11.05 pounds for a typical worker.

Over a third of zero-hours workers are aged 16-24, and one in five said they were in full-time education, the ONS said.

But nearly half the increase seen in zero-hours contract workers over the past year was among workers aged between 25 and 64, raising concerns about job stability for people likely to have dependants.

"It is concerning that much of the growth is among older workers, some of whom might struggle to balance their family budget if their income varies as a result of having no guaranteed hours," Conor D'Arcy, an analyst at the Resolution Foundation, a think tank, said.

"Banning zero hours contracts is not the answer, as it punishes workers who genuinely prefer the flexibility they offer," D'Arcy said. "But the part they play in the wider issue of insecure working suggests that they need far closer, and more measured, scrutiny."

The ONS said it was likely that some of the increase in the number of people reporting they were working on zero-hours contract was due to increased awareness of the issue.

(Reporting by Laura Gardner Cuesta, editing by William Schomberg)