The Government Equalities Office expected data from around 9000 organisations.
The mean pay gap is calculated by adding up hourly wages paid to full time male employees, and full time female employees.
Some companies with large gaps have said they pay women and men in similar jobs the same amount. The figure is above that of rival HBoS, which sits at 26.4 per cent.
"Their data is being skewed by a high number of really highly paid people at the top", said Suzanne Doyle-Morris, an expert and consultant on gender issues in the workplace.
Groundscare equipment manufacturer Ransomes Jacobsen in Ipswich had a median gap of -7%, meaning women's average pay was higher than men's.
"As we're socializing teenagers into the workforce, one unintended effect is that we're socializing them into the problems of the workforce".
"To combat this, we're focused on encouraging more women into tech roles".
Companies who do not provide their figures could face legal action including court orders and fines.
Fawcett recommends that the law should be changed so that civil penalties for non-compliance are introduced, which along with additional resources for the EHRC would enable them to have a more immediate impact with enforcement activity.
Video What does the gender pay gap data tell
"By finding out what their colleagues earn they are then in a position to challenge any pay inequality; it is much more common than people realise".
As the gender pay gap data revelations are all coming to a head today, companies and policymakers alike have been trying to track when gap widens between genders - but as a recent study has shown, that gap appears disappointingly early.
Headed up by Cathy Southgate, Johnston Press Group Financial Controller (with oversight from Chair of the Board, Camilla Rhodes) the Gender Pay Review Group will aim to reduce the gender pay gap within five years by finding solutions to the three main areas of concern - tackling the mean gender pay gap and having more women in management and Senior Management Team roles.
The numbers are at times uncomfortable reading in an industry that has traditionally been male-dominated, though female executives are increasingly being appointed to senior roles.
The managed hosting provider has a baseline mean gender pay gap of 0 per cent.
What if employers don't publish their data?
The deadline has passed for companies to report their gender pay gaps, and the results are in.
"Employers that do not comply will risk facing legal action from the Equality and Human Rights Commission".
The BBC, which has a gap of nine per cent, has already declared it is working to improve the balance in earnings and representation.
Unlike Amazon, however, Apple's statement cites both mean and median statistics and admits that by some measures its female employees are worse off.
Director General Tony Hall said: "This is a fantastic project that is already driving change".
Dragon Resupply Mission (CRS-14)
When the capsule leaves in May, it will have NASA's old Robonaut 2 on board, a humanoid robot that's been on the ISS since 2011. The technology can be used to 3-D print metal parts, but it's trickier to do in zero gravity than it is on Earth.