Union opposition to an early-retirement deal is going to lead the Parks Department to lay off 465 workers after the summer, The Post has learned.

To cut costs by $29 million — and avert the layoffs — Parks Commissioner Adrian Benepe this year proposed speeding retirements by allowing anyone who left the payroll by a certain date to return for six-month seasonal stints in each of the next three years.

City officials said District Council 37, which represents parks workers, wouldn’t accept the offer unless it was accompanied by a guarantee the agency would halt all layoffs.