The ruling by Mr Justice Morgan could cost Lloyds £150million and the pension industry collectively around £20billion.
Employees Angela Sharp, Judith Cain and Susan Dixon by their trade union BTU, claimed they faced sex discrimination because their guaranteed minimum pension increased at a lower rate than their male colleagues.
Around 35,000 Lloyds pension scheme members are likely to receive a minimum refund of £500.
Of those 35,000, about 8,000 could receive at least £3,000 over their retirement.
The outcome of the case will have profound implications for both public and private sector pension schemes in all industries across Britain.
Up to five million people in 6,000 private sector schemes, the vast majority of whom are female, will benefit from the ruling.
The Government has estimated that the cost of the case at £10billion-£20billion.
BTU general secretary Mark Brown said: “This landmark judgment resolves this pension discrimination issue for good and will bring equality to millions of women across the country.
“It’s simply unacceptable that 48 years since the passing of the Equal pay Act in 1970, we are still fighting for equal treatment in the workplace.”
Guaranteed minimum pensions (GMPs) were introduced in 1978 so employers and staff could pay lower national insurance if they replaced some of the state pension.
They could “contract out” of the State Earnings Related Pension Scheme (Serps) as long as they offered to replace this part of the state pension with benefits at least as good as a stated minimum level.
Women and men had different state pension ages, so originally company schemes did too
This could prove damaging to pension schemes which are in a parlous financial position
The employers and members paid lower national insurance and almost all final salary schemes contracted out.
Women and men had different state pension ages, so originally company schemes did too.
But in 1990 the European Court of Justice ruled that men and women must be treated the same and schemes equalised pension ages, often at 65.
But rules governing the GMP element were set in law and as state pension ages were still different, equalisation was not applied to the way GMPs were calculated, so women lost out.
Sir Steve Webb, a former pensions minister and now director of policy at the mutual Royal London, said: “This ruling finally provides clarity over this contentious issue.
“Schemes will need urgent help from Government and regulators to know the best way to respond.
Around 35,000 Lloyds pension scheme members are likely to receive a minimum refund of £500
“Members of company schemes could collectively receive a multibillion pound windfall, but the complexity of making the necessary calculations means that members will not be receiving cheques any time soon.”
The High Court confirms GMP equal-supportedisation is necessary and three different methods can be used.
However the costs of compliance is daunting and pension schemes may have to spend billions checking their GMPs and increasing pensions, experts say.
Another ex-pensions minister, Baroness Altmann, said: “Many schemes do not have accurate records for GMPs and have been trying to reconcile their data with HMRC since 2016 when contracting-out ended.
“However, as well as paying arrears, this could prove damaging to pension schemes which are in a parlous financial position.”
Anna Rogers, partner at ARC Pensions Law, said: “Schemes also have to pay interest on arrears, which is important given that the relevant service dates back over 20 years.”
David Everett, of lawyers Lane Clark & Peacock, said: “Trustees and scheme sponsors will not relish having to recalculate benefits going back almost 30 years, but the ruling does give clarity that something needs to be done.”
Lloyds Banking Group said: “The group welcomes the decision made by the court and the clarity it provides. The group and the pension scheme trustee will be working through the details in order to implement the court’s decision.”
The Department for Work and Pensions said: “We are due to publish guidance shortly.”
Mr Justice Morgan of the Queens Bench
COMMENT BY BARONESS ALTMANN
THIS ruling could have enormous ramifications for most final salary-type pension schemes.
The case revolves around guaranteed minimum pensions.
In 1978 when GMPs were introduced, men and women’s state pension ages were different and most company schemes followed the same practice, with women able to receive their company pension five years before men.
A 1990 European Court of Justice ruling led to schemes equalising pension ages but as state pension ages were still different the equalisation did not apply to GMPs.
GMPs have to increase each year, but the rate of revaluation depends on the year in which each member reached pension age.
The women’s GMP started earlier than men’s and the rules relating to later years allow men’s GMPs to be increased at higher rates. Today the High Court ruled that the GMPs do have to be equalised.
Baroness Altmann in her office at The House of Lords
However, there are two major problems. Firstly, data records are often incomplete and inaccurate.
Secondly, the actual calculations are so complicated that it is almost impossible to get it right.
Contracting-out ended in 2016 and all schemes must reconcile GMP records with the Inland Revenue to ensure they are correct.
In reality, most will see very little change in their pension payments.
However, pension schemes may have to spend billions checking GMPs and increasing pensions, as well as paying arrears.
Baroness Altmann is ex-pensions minister