If you've been anywhere near social media in the last few hours, you'll be well aware that today is Equal Pay Day.
It said that because the gap was widest for older women, it meant the United Kingdom was actually "going backwards" in its pay parity efforts.
Send some of them an email and you'll receive an automatic reply telling you they are "Out of Office" until the end of the year - even though they are at work.
Mackay pointed out that over any ten year period since the stock market began, stocks have done better than cash nine times out of 10, meaning women have continued to miss out on these returns. With all the intelligence we have in the world, the gender pay gap is a solvable - and should be a solved - problem.
"I'm doing it to support those on lower wages - I work for a lot of elderly people whose [female] carers are on such poor wages", she says.
"There are no excuses, employers now need to get on with the job of publishing their pay gap and pledge to improve workplace equality". "It should matter to everyone #equalpayday".
The society says the pay of younger women in particular is falling behind that of men, claiming the drive to equalise pay is going backwards and will now take 100 years to close.
The group also said that although the gap is wider for women in their 50s, it is also growing among younger women from 1.1 per cent in 2011 to 5.5 per cent this year.
Chief executive Sam Smethers said that was extremely worrying.
"Discrimination and sexual harassment can be hidden and more common than they think".
A women's website, dating app and several advertising agencies are among the organisations who have put their out of office on "for the rest of the year" to mark the fact that today, women effectively stop earning in 2017.
What can women, men, companies, governments, educational institutions and the HR industry do to make change?
Speaking to the Radio Times about the issue in October, Montague said the disparity in pay was "professionally damaging".