The CCOO union, one of Spain's largest, said it will seek in the next round of negotiations starting in January clauses allowing revisions if inflation were to accelerate further, in addition to the minimum 18% increase over the next four years.

FASGA, another union, is demanding an increase of almost 20% over four years.

"The inflationary environment is already generating a loss for workers' purchasing power that has to be addressed and corrected in the next agreement," CCOO said in a statement.

ANGED, the business association representing some of the biggest retailers in Spain declined to comment as negotiations with the unions has not yet started.

Retailers, also under pressure from the Spanish government to keep prices down, have said they already are making "an extraordinary effort" to avoid passing on rising costs, such as energy, to consumers as much as possible.

European retailers and businesses in general face growing demands from their workers for double-digit wage increases to match or surpass inflation, which stands at or near three-decade highs. Higher wages, in turn, are likely to lift prices of goods and services further down the road.

Spanish 12-month inflation stood at 10.5% in August.

(Reporting by Corina Pons; Editing by Inti Landauro and Alexander Smith)