A threatened strike by 12,500 janitors in Massachusetts and Rhode Island was averted Wednesday when the union representing the contracted cleaners reached a tentative agreement with an association of the region’s largest cleaning contractors.

The agreement offers the janitors wage increases of around 20 percent over the course of a four-year contract, according to the Service Employees International Union.

The deal also will convert 500 part-time positions in Boston and Cambridge to full-time jobs over the course of the agreement, letting many union members who are cleaning biotech and higher education sites to access employer-paid health benefits for the first time, the union said.

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“This accord secures the largest wage increases we have ever negotiated, allowing members who worked through the pandemic to keep up with historic inflation and some of the highest prices for housing in the nation,” said Roxana Rivera, the head of the union in Massachusetts and Rhode Island.

The agreement averts a strike, which could have begun on Thursday after the current agreement was set to expire with the 60 employers represented by the Maintenance Contractors of New England.

Michael White, president of the Maintenance Contractors of New England, said both sides were relieved to avoid a strike.

“It’s a fair agreement for both sides,” White said.

A call to a representative of the contractors’ association was not immediately returned on Wednesday.