Pensioners who work for firms like BT, Marks & Spencer and Ford could lose billions if government retirement changes go through, experts warn.
The issue is that almost 450,000 people in these firms' pension schemes, with £83billion in their retirement pots, will have smaller pensions from 2030.
The Government has already said it does not plan to compensate people who lose out.
Workers in many pension schemes have retirement pots that go up every year in line with the retail prices index (RPI) - one of two main inflation measures.
RPI is almost always higher than its main rival, consumer prices inflation (CPI).
But the Government wants to change how RPI is worked out, bringing it more in line with CPI.
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But doing so will wipe billion of pounds off defined pension pots for affected workers, according to BT, M&S and Ford UK pension schemes.
The average male worker aged 65 with a pension affected by the change will see yearly pension cash fall from £6,300 a year to £5,500, the Pensions Policy Institute has said.
For women the figure is £6,200, falling to £5,300.
Three pension schemes have brought a legal challenge against the government and the UK Statistics Authority (UKSA) at the High Court over the issue this week.
A decision is expected in around three months.
The High Court heard that anyone in a pension scheme where increases are linked to RPI will be hit by the changes.
Affected retirees will get 4% to 9% cut off their pensions - with women hit worst as they tend to live longer.
Around 82,000 BT pensioners will lose £34,000 each on average, the court heard - wiping out more than £2billion in pension cash.
The changes are due to take place from February 2030.
BT, Ford UK, M&S and HM Treasury have been approached for comment.
A spokesperson for HM Treasury would not comment as the case is not yet over.
However, the argument put to the court by the chancellor and UKSA said RPI has "never been subject to a set or immutable methodology".
The argument added that the Chancellor was "expressly advised" that the decision would hit pensioners, but that he decided to do it anyway.
"The decision is unimpeachable," the Chancellor said in court papers.
A joint statement from the pensions schemes of BT, Ford and M&S said: "It is estimated that over 10million pensioners, through no fault of their own, will be poorer in retirement either from lower payments or lower transfer values as a result of the effective replacement of RPI with CPIH.
"Women will suffer the most from this change as they typically live longer."
However, lowering RPI is good news for students and commuters.
That's because the Government links things like rail fare increases and student loan repayments to RPI.
Increases to a string of taxes are also linked to RPI - including car tax, air passenger duty and fuel duty - potentially seeing savings there.
Many contracts for mobile phones also allow "inflation linked" rises each year - meaning people could also save money that way too.
What's currently linked to RPI?
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Final salary pension payments
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Income from index-linked annuities
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Income from some index-linked bonds
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Regulated rail fare rises (and wage negotiations for rail workers)
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Mobile phone tariff price rises (the maximum rise allowed without triggering your right to leave the contract early)
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Vehicle excise duty (better known as car tax)
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Air passenger duty rises
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Tobacco and alcohol duty rises
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Interest on student loans
