The MTA shelled out $1.4 billion in overtime alone last year — yet had no way to guarantee its employees actually worked the extra hours they claimed, the agency’s inspector general said Friday.

Instead, many of the MTA’s supervisors “relied entirely on the honesty of employees” — creating an “honor system” rife with opportunities for fraud, IG Carolyn Pokorny concluded after a review of the 75 overtime-collecting “high-rollers” among the agency’s 70,000-person workforce.

Of particular concern were transit workers who spent their overtime hours away from their regular work site and the supervisors responsible for approving their timesheets.

The majority of supervisors interviewed for the report told the IG’s office they “do not or cannot” confirm that the off-site OT was assigned or worked.

“It was common practice for many [supervisors] to rely entirely on the word of the employee,” Pokorny wrote in her report summary.

One subway supervisor who never once attempted to verify one particular high-roller’s off-site overtime told the IG’s office that its review inspired him to be more thorough — and ultimately scale back that employee’s hours.

In another instance, a group of Long Island Rail Road employees collected overtime pay for off-site work on behalf of an outside contractor — who did not keep any record that those hours had actually been worked.

“The absence of a proper system to verify … overtime hours could create opportunities for employees to claim overtime that was not worked or even assigned to them without being detected,” Pokorny said, calling the lack of effective oversight a “critical failure.”

Friday’s report is the latest salvo in Pokorny’s war on the soaring MTA overtime costs and overtime fraud exposed by The Post this year.

The money that the agency spent on overtime in 2018, $1.4 billion, was 8.5 percent of its budget — compared to just $849 million in 2014.

A third-party report released in August pinned the outrageous OT costs on archaic timekeeping methods.

The MTA began rolling out biometric timekeeping clocks system-wide this summer, but Pokorny said more must be done to integrate the system with the agency’s payroll process.

“It is critical that MTA management use modern timekeeping methods that allow supervisors to easily and reliably verify claimed overtime,” she said in a statement. “Riders and taxpayers deserve management accountability.”