Russell George, a Powys county councillor and Montgomeryshire AM, said in a debate in the Senedd that the council has suffered from years of unfair funding settlements.
Mr George said: "For eight years in a row, Powys County Council has received one of the worst local government funding settlements from the Welsh Government of all 22 local authorities in Wales.
"It is now essential that the Welsh Government reforms the formula which it uses to decide how much each council receives.
"The current system is fundamentally flawed as it doesn't properly take into account the challenges of delivering vital public services over large rural areas of Mid Wales."
A recent special budget seminar held by Powys County Council has confirmed that the under-funding of Powys has left the authority facing a £27 million funding shortfall over the next three years and that the Powys budget would have been £16 million higher if the county had received the Welsh average.
Mr George added: "The current system provides councils in urban south Wales with far more attractive financial settlements while the largest, most rural councils of Powys, Ceredigion, Monmouthshire and Pembrokeshire are among those with the worst cash settlements.
"Over the last nine years, Powys has received a 0.5 per cent increase in funding compared with the Welsh average of 9.8 per cent.
"Cardiff has had a funding increase of almost 19 per cent during the same period.
"We in Powys deserve much better and I would urge all people to sign a petition on my website."
To sign Mr George's petition, which is calling on the Welsh Government to address the funding issue, visit russellgeorge.com/fair-funding