Women reaching the age of 60 face a cut in benefits based on the level of their old age pension, while men receive the full amount up to their pensionable age of 65.
The Social Security Commission, an independent Crown body headed by a judge who rules on appeals and matters of principle, has declared Britain's stance on invalidity benefit discriminatory and urged equal treatment.
Peter Lilley, the Secretary of State for Social Security, and Whitehall officials decided to contest the commissioner's ruling in a case that is likely to be heard in the Court of Appeal before the end of the year.
The Government has circulated all social security offices instructing them to withhold the extra sums, but has indicated that it would pay out any arrears should the court reject its arguments.
However, the department's policy branch has conceded it might relent if the 'customer was terminally ill or if he, or she, was experiencing dire financial hardship'.
But a letter advising claimants of the present state of play is less generous. It states: 'If the Court of Appeal decides that the benefit should not be increased, you will continue to be entitled to the rate of benefit you are getting now, unless there are other changes which affect the rate. You do not have the right to appeal against this decision.'
Women who retire on health grounds before their 55th birthday, and men before their 60th, are entitled to a basic invalidity benefit plus another element.
It may be an invalidity allowance, or an additional component, fixed by the level of National Insurance contributions in the last tax year before they stopped work.
When the women reach statutory retirement age they become eligible for the standard old age pension, but alternatively may opt to continue receiving invalidity benefits instead.
If they choose the latter and were in receipt of the additional component, under existing regulations they will find this component withdrawn and replaced by the invalidity allowance, a smaller amount.
On the other hand, men on invalidity benefits will not reach retirement age until their 65th birthday and are able to continue claiming the additional component until then.
The discrimination results from the difference in retirement age between men and women, a discrepancy which the Government has already committed itself to equalising, though this may take many years.
The Department of Social Security maintains that the European directive on equal treatment for men and women allows for different pensionable ages, including additional benefits.
It was on that basis that it planned to contest the Social Security Commissioner's ruling that UK law is discriminatory.
Sally Witcher, a campaign worker for Disability Alliance, an umbrella group, attacked the Government's stance, accusing it of going to 'any lengths' to avoid making payments.
'They will fight this all the way and then if they lose they will find some other way of stopping it so that they don't have to backdate it,' she said.
'They have done this numerous times with social security benefits. It's not particularly surprising, but it's sad for the individuals who actually need the money.'