As Mayor de Blasio considers whether to offer teachers retroactive pay raises, the conservative Manhattan Institute would like him to know that, in fact, many teachers’ salaries increased during the four years they have gone without a contract.

Echoing statements that then-Mayor Bloomberg made last year, the think tank argues in a new brief that teachers earned $1.2 billion since 2009 in automatic salary increases tied to years of employment, which remain in effect even after a contract expires. If pension contributions and payroll taxes are included, the “total compensation” increase was $2.1 billion, the brief says.

Manhattan Institute analysts in the past have argued that teachers should be paid based on performance instead of seniority, and that teachers’ pensions should be reduced in favor of higher salaries.

The group’s latest post urges the new mayor to insist in negotiations with municipal unions that automatic pay hikes should freeze until new contracts are in place.

“This will give the UFT and other unions a stronger incentive not to spend years ‘working without a contract’,” the author writes.

It’s worth noting, however, that veteran teachers receive less frequent automatic raises. That means that many teachers did not enjoy pay bumps during the past few years.