The Absolutely Fabulous legend insisted the Prime Minister must hold a face-to-face meeting with Anne Puckridge when she arrives in the UK next month.
Ms Puckridge, who served in all three branches of the Armed Forces, is making the arduous 4,400 mile journey to London from her home in Canada just days before her 100th birthday in December.
She is among almost half a million retired Britons living overseas whose state pensions do not rise with inflation.
Their weekly payments were frozen on the day they left the country, meaning many are missing out on thousands of pounds a year compared with their domestic counterparts.
The decades-long policy means they get less than half the amount of those living in this country.
Some of these pensioners live in poverty while many others are struggling to get by.
Ms Puckridge, who lived and worked in the UK until she was 76, believes she has lost out on an estimated £60,000 since moving to Canada in 2001.
Actress Joanna, 78, said: “The Prime Minister is a decent and honourable man. That’s why I’m hopeful that he will push back against the raft of ‘whys and wherefores’ that dry officialdom will no doubt put before him urging him to avoid meeting Anne when she makes her epic journey in December.
“Surely there’s no more powerful way of demonstrating that he recognises her service, and those she represents, than to grant her a few minutes of his precious time.”
Her rallying cry comes after campaigners launched new petition calling on the government to end the injustice.
Sir Keir is already facing the wrath of pensioners for cruelly snatching away the winter fuel payment.
Labour promised to uprate pensions for British expats living overseas in its 2019 general election manifesto but didn’t include the pledge this time around.
The policy has been left unchanged by successive governments for around 70 years.
In 1955, British state pensions became payable anywhere in the world – but were not inflation-linked.
Reciprocal arrangements with only certain countries to uprate pension payments have been made in the ensuing decades.
But there are still around 150 countries where there is no agreement, including many Commonwealth nations like Canada, Australia, South Africa and New Zealand.
Campaigners argue the cost of changing the policy would be around £300 million over five years and just £50 million in the first year as it would start from the day any deal was signed, rather than being backdated.
The basic state pension is now worth £169.50 per week.
End Frozen Pensions campaigners say this means the 40% of all British pensioners living outside the UK who are hit by the policy are losing out on £3,085.16 a year if they retired in 2013 and up to £7,042.36 a year if they retired in 1983.
John Duguid, Chair of the End Frozen Pensions Campaign that represents affected pensioners across the world, said: “Joanna Lumley’s support will mean the world to Anne and all the other affected pensioners.
“We hope it will be an inspiration to the wider public across the UK and beyond to get behind our call for the Prime Minister to meet Anne.
“I hope the Prime Minister and those around him will not see this brave campaigner, of impeccable standing and with such a record of service to the country, left on the pavement. We are sensing a groundswell of support from across Parliament and the world that a meeting is the very least she deserves.”
Ms Lumley’s plea comes after 32 MPs signed a letter urging the Prime Minister to find time to meet the veteran.
Tonia Antoniazzi, the Labour MP for Gower and long-time advocate for the ending of the frozen pensions policy said: “The more I hear about this policy the more indefensible it becomes. For decades now, successive Governments have hidden behind figures from the DWP suggesting that righting this great wrong would cost billions. That is simply not the case.
“The pensioners are only asking for uprating from this point going forward. Campaigners estimate that uprating on such a going forward basis would cost about £50 million in the first year, a tiny proportion of the overall pensions budget.
“In Treasury terms that’s a small price to pay to address such a great and long-standing injustice that’s hit generations of pensioners. In all conscience, how can any of us look someone like Anne Puckridge in the eye and not feel that she deserves better.”
Christine Jardine, Liberal Democrat member for Edinburgh West said: “Anne’s story, and the overall injustice that it so powerfully illustrates has touched me to the core. This is something the Government could, and should, change.
“That’s why I’m appealing to all my parliamentary colleagues, irrespective of party allegiance, to get behind the call that the Prime Minister should at least agree to meet Anne with a view to addressing this issue.
“We come into politics to make positive change on what matters to people. And this is a change that needs to happen.”
A government spokesperson said: “We understand that people move abroad for many reasons, and we provide clear information on gov.uk about how this can impact their finances in retirement.
“The International Pension Centre is a source of advice for people who are already retired.
“The Government’s policy on the uprating of the UK State Pension for recipients living overseas is a longstanding one of more than 70 years and we continue to uprate state pensions overseas where there is a legal requirement to do so.”
