Scotland's three biggest companies pay women up to a third less than men on average, it has emerged.

The Royal Bank of Scotland, Lloyds and SSE are the largest firms registered north of the border, with more than 20,000 employees each.

RBS has reported an average hourly pay gap of 36.5%, meaning women take home 64p for every £1 men earn.

Financial firms have reported among the worst divides, which gender expert Dr Suzanne Doyle-Morris puts down to culture.

Dr Doyle-Morris, from the InclusIQ Institute in Edinburgh, said: "Most of these organisations don't have cultures where women want to stay in the long-term."

More men received bonuses at RBS than women and men were given larger bonuses on average.

"The sector as a whole heavily relies on bonuses and they're rife for bias - most people have to negotiate for them and men tend to be more successful at that," Dr Doyle-Morris explained.

"It's not a culture that's welcoming to women."

RBS has set itself a target of filling at least 30% of senior roles with women by 2020.

A spokeswoman said: "We're making good progress towards correcting this imbalance."

"By continuing to challenge ourselves we are confident we can significantly move the bank towards a 50/50 balance at all levels and substantially reduce our gender pay gap," she added.

Lloyds Banking Group - which includes Bank of Scotland - has reported a 32.8% pay gap, still well above the UK average of 18.4%.

Lloyds has committed to having 40% of senior roles filled by women by 2020.

SSE's gap is closer to the national average at 19.3%.

Dr Doyle Morris said it is likely the result of problems attracting women to jobs in science and engineering.

John Stewart, SSE's director of HR, said: "Genuine transformation for SSE, and across the UK's labour market, will require meaningful societal changes as well as improvements at organisational-level.

"SSE is committed to being a leader for driving change in both of these areas."

SSE was among the first Scottish companies to report its gender pay gap in 2016.

Other prominent Scottish companies which have reported their pay gaps include Lloyds Banking Group (32.8%), Scottish Power Energy Retail (30.5%), Wood Group (23.1%), Diageo (16.7%) and BrewDog (2.8%).

Media companies which have stepped forward include STV (17.3%), Herald owner Newsquest Herald and Times (17%) and Daily Record owner Scottish Daily Record and Sunday Mail (15.5%).

The gender pay gap is the difference between the average hourly earnings of men and women.

It is different from equal pay as it has been illegal to pay men and women differently for doing the same job since 1970.

A gap is often the result of men occupying most of the top jobs at an organisation.

We have used the median average because it is not as affected by extreme values like the high earnings of a small number of people at the top of an organisation. This method is also favoured by the Office for National Statistics.

However, because high-earners are predominantly male, the mean is an important measure of women's experiences in the labour market. It also makes international comparisons easier.