Labour has formally backed the campaign against the freezing of state pensions for some British expats because it is "contrary to natural justice", Jeremy Corbyn said.
UK pensioners who live in certain countries like Australia and Canada have their state pension frozen but those living elsewhere see the amount they receive uprated annually.
The freeze applies to Britons who live in one of the 120 countries that do not have a reciprocal agreement with the UK requiring that uprating takes place, and campaigners estimate that more than half of expat pensioners do not get an increase.
Mr Corbyn, a longstanding supporter of the campaign against the freeze, will now seek a vote in Parliament on regulations which will exclude some overseas pensions but uprate all others by 2.5%.
The party has just a week to secure a House of Commons debate and vote, as it rises for Easter recess on March 30 and the uprating regulations come into force on April 10.
The Labour leader said: "This is a chance to make an historic change to our pension system and end the arbitrary discrimination against some British pensioners living overseas.
"It is contrary to natural justice for some pensioners living abroad to be left behind while others have their pensions increased in line with inflation.
"The next Labour government will treat all our pensioners equally, wherever they live, and ensure that overseas pensions are levelled up not down when Britain leaves the EU."
John Markham, chairman of the International Consortium of British Pensioners, said Labour's support represented a "significant step" in the campaign to unfreeze expat pensions.
"Frozen pensions are the dirty secret of successive governments, who have been content to ignore pensioners they felt were out of sight out of mind, regardless of the implications," he said.
"Many expat pensioners are just as reliant on their state pensions as those living in the UK.
"Freezing their pensions leaves recipients with dwindling incomes, deprives them of their prized independence and leaves many in dire poverty towards the end of their lives.
"I am glad this national embarrassment is finally getting the attention it deserves."
A Department for Work and Pensions spokeswoman said unfreezing state pensions for relevant expats would cost more than £500 million a year.
The spokeswoman said: "The state pension is payable worldwide and is uprated in the European Economic Area and where there is a reciprocal agreement in place. Uprating the state pension where it is not currently uprated would cost more than half a billion pounds a year."