Tesco has revealed a gender pay gap of 8.7% in its UK business, making it the first of the major grocers to report the figures ahead of an April deadline set by the government.

All companies with over 250 staff will now be required report information on their median and mean gender pay gaps as well as differences in bonuses.

After analysing pay data of more than 225,000 employees in its UK business, Tesco said the median gender pay gap – identifying the wage of the middle earner – was 8.7%, and the mean (or average) gender pay gap – taking into account low and high earners within the organisation – stood at 12%.

Meanwhile, the group’s median bonus gap was much higher at 27%, with mean bonus gap coming in at 42.6%.

Tesco’s Gender Pay Report 2017 stated that the analysis had found the gap between women’s and men’s pay was due to “career and lifestyle choices” with more male workers picking up shifts at times that offer premiums, such as nights and bank holidays. It also cited the fact that it had more male members of staff in senior positions.

The Tesco report stressed: “For the same role, regardless of gender, all our hourly paid colleagues are paid the same hourly rate and premium hourly rate. When we remove the premium payment from the calculation, then the gender pay gap reduces even more significantly to a median of just 2.7%.”

Tesco’s UK Chief Executive Matt Davies said: “While we’re pleased that our gender pay gap of 8.7% is significantly below the UK median [18.4%], we want to close the gap altogether.”

Regarding the relative lack of women in senior roles, Davies said: “I’m pleased that we are making progress in this area and exceeded our own target of 25% women on the board by the end of 2017. But we know there is more to do when it comes to other leadership roles in our business. So earlier this year we signed up to the 30% Club, which encourages businesses to achieve a minimum of 30% female representation in senior leadership roles by 2020.

“We believe that long-term, sustainable change will take time. But we also believe it is the right thing for our business and essential for our long-term future.”

The publication of the Tesco data comes just weeks after legal proceedings began against the retailer for an equal pay claim that could cost as much as £4bn in compensation payments.

Law firm Leigh Day launched the legal action on behalf of nearly 100 female shop workers amid claims they earn as much as £3 an hour less than male warehouse staff despite the value of the work being comparable. If the legal challenge demanding parity is successful, thousands of shopfloor staff could receive back pay of up to £20,000 each.