Mayor de Blasio this week opened up a can of worms by denying the official fiction that the Rent Guidelines Board is an “independent” agency he doesn’t really control.

The mayor admitted the reason the board twice voted to freeze stabilized rents citywide — the first time it had ever done so — is that he “instructed” its members to do just that.

In the past, de Blasio has insisted that suggestions (including ours) that the board is his political tool are “ridiculous.” Oops.

He also admitted that the board, at his bidding, completely changed how it has evaluated rent hikes since its 1969 birth.

“As you know,” de Blasio told a conference call with senior citizens, “for two years now we’ve had a rent freeze for all rent-stabilized tenants in the city. That’s never been done in history before.

“This happened . . . because I instructed the Rent Guidelines Board — I name the members — and I instructed them not to follow the biases of the past.”

Those “biases” being a mandate to balance the needs of tenants and landlords. Instead, the board clearly paid less heed to landlords’ operating costs.

Of course, rent hikes have rarely reflected actual costs. In his first year as mayor, the de Blasio-controlled board voted a then-record-low 1 percent hike, though costs had risen that year by 5.7 percent.

As we’ve noted many times, rent freezes aren’t cost-free. For landlords, they represent a cut — and rent regulations in general make middle-class housing unaffordable.

A judge will rule this month on a landlords’ lawsuit challenging the most recent rent freeze as politically motivated. Looks like the mayor just provided some critical evidence.