Gurkhas won the right to settle in the UK after a high profile campaign
Campaigners representing retired Gurkhas are going to the High Court later in a battle over their pensions.
The appeal concerns 24,000 veterans who served before July 1997 and are entitled to only a third of the monthly payment of their UK counterparts.
The British Gurkha Welfare Society is seeking a judicial review against an earlier ruling in the case.
The Ministry of Defence says Gurkha pensions are paid over a longer period and amount to the same final level.
It comes after Gurkha veterans with a minimum four years' service won the right to settle in Britain after a high-profile legal challenge led by actress Joanna Lumley.
The latest case followed an offer by the MoD in March 2007 to transfer Gurkhas' pensions into one of the mainstream armed forces schemes.
The change did not apply to those who retired before 1997, the year the Gurkhas base formally transferred from Hong Kong to the UK.
According to the British Gurkha Welfare Society, those who retired before 1997 have an "inferior monthly pension".
It added that there were also about 7,000 Gurkha veterans who served for less than 15 years receive no pension at all and around 5,000 veterans and widows who currently rely heavily on charity from its own scheme.
British Gurkha Welfare Society general secretary Chhatra Rai said: "This is above all a moral issue as the majority of Gurkha veterans in this group are now becoming increasingly old and fragile.
"Most of these Gurkhas are therefore not able to work in their old age to supplement their pension."
Some 94 MPs have supported a House of Commons motion from Labour's George Howarth calling for improved pension rights for the Gurkhas.
He said: "The fact that this small but significant group of veterans are still discriminated against shows that there is still more to do to ensure that the Gurkhas are treated properly and fairly."
Another supporter, Conservative MP Ann Widdecome, said: "The Gurkhas have always been an integral part of the British Armed Forces, fighting the same wars and carrying out the same duties as British soldiers.
"It is an injustice to give these veterans a pension based on their country of origin instead of the country in whose Army they loyally served."
A Ministry of Defence spokesman said: "Over the course of a retirement, Gurkha soldiers will receive the same amount of pension as their British counterparts.
"Gurkha pensions can be paid from the age of 33, 20 years before British pensions begin. That means a Gurkha rifleman will already have received approximately £40,000 by the time he is 60."