The social association VdK is planning a model lawsuit against unequal treatment in the new energy flat rate.
"Poor pensioners, but also caring relatives, people who receive sickness or parental benefits, get nothing," criticized the VdK chairwoman Verena Bentele in the "Bild am Sonntag".
This violates the principle of equality.
"That's why we plan to sue against this injustice," said the head of the association.
The energy lump sum of 300 euros gross for employees who are subject to income payments, which is to be paid out in September, is part of the federal government's relief packages.
The traffic light coalition had put together this in view of rising prices for energy and other goods, and the Bundestag and Bundesrat approved the measures last month.
The measures also include a fuel discount, a one-off payment for Hartz IV recipients of 200 euros, an immediate surcharge for children in low-income households of more than 20 euros per month and a one-off child bonus of 100 euros per child for all families.
The German Federation of Trade Unions, among others, is also calling for improvements to the relief package for pensioners and students.