The UK government said 75% of farms will not be liable for inheritance tax.
But farming unions said the changes “will bring the majority of Wales’ family farms into the scope of this tax”.
The Treasury did not have an estimate of the impact on Welsh farms.
Across the UK it said it expected 2,000 estates to be affected from 2026-27, with around 500 of those claiming agricultural property relief.
Liz Saville Roberts, Plaid Cymru’s party leader in Westminster and MP for Dwyfor Meirionnydd, said she was concerned the change would lead to farms being broken up and sold off.
“One factor that hasn’t been considered… is the effect this is likely to have on tenant farmers.
“If you’ve got large estates being hit by death duties, they will then sell up.
“Tenant farmers, they could then have their land sold out from underneath them,” she told BBC Radio Wales’ Sunday Supplement.
Andrew RT Davies, the Conservative’s leader in the Senedd, said UK Labour had given assurances last year that there would not be any such changes, allowing “family farms to continue to be passed through the generations”.
“This is a devastating change, a brutal betrayal and they should hang their heads in shame,” he said.
