Global delivery giant UPS is accused of underpaying its seasonal workers, vital to meeting the demand of the annual glut of online shopping, according to a lawsuit.

The New York attorney general's office said in the lawsuit filed on Monday that UPS, which claims to deliver 22.4 million packages daily, "commits wage theft against these workers in myriad ways".

"UPS has repeatedly and persistently failed to pay seasonal delivery workers for all time worked," prosecutors alleged, suggesting thousands of past and present workers are affected.

The filing claimed UPS workers were not being paid for their labour before and after shifts as well as during meal breaks while alleging UPS's timekeeping "introduced and compounded" errors.

In a statement, attorney general Letitia James's office said "these unlawful practices deprived workers of millions of dollars and is asking the court to order UPS to pay restitution for current and former seasonal workers whose wages were unlawfully withheld."

"Instead of compensating these workers fairly for their labour, UPS has played the Grinch," James said in the statement referencing the Dr Seuss character who famously stole Christmas.

"UPS takes all accusations of wrongdoing seriously and denies the unfounded allegation of intentionally underpaying UPS employees," the company said in a statement.

"We offer industry-leading pay and benefits to our more than 26,000 employees in New York, and we remain committed to following all applicable laws." (AFP)