* Q1 operating loss Y10 bln, vs Y7.4 bln profit year earlier
* Lowers annual operating profit estimate to 25 bln yen
* Books a further 5.2 bln yen charge due to unpaid wage bills (Adds executive comment)
TOKYO, July 31 (Reuters) - Parcel delivery firm Yamato Holdings Co booked a second straight quarterly loss and cut its annual outlook on fresh charges for unpaid overtime - its woes underscoring a deepening labour shortage that is gripping corporate Japan.
The nation's No.1 parcel delivery company has struggled to meet a surge in demand as e-commerce booms. At the same time, the available pool of workers has shrunk amid a rapidly ageing population.
That quandary has found Yamato posting a charge in addition to one last financial year after finding that thousands of workers had not been paid overtime.
The company known for its black cat logo reported a first-quarter operating loss of 10 billion yen ($90.5 million), compared with a profit of 7.4 billion yen for the same period a year earlier.
Taking a 5.2 billion yen charge for unpaid wages, it also cut its forecast for annual operating profit by 17 percent to 25 billion yen.
Lacking sufficient workers, Yamato has been forced to increase its use of third-party delivery firms and announced plans to hire 10,000 workers.
It has also pledged to improve worker conditions, hiked prices for individual customers and is seeking to renegotiate delivery contracts with retailers so the number of delivery time slots are reduced and redeliveries due to customer absence are avoided.
Yamato's home delivery parcel volumes have grown 60 percent over the last 10 years, reaching some 1.87 billion parcels for the year ended in March.
While the company's has pledged to reduce parcel numbers by 80 million this financial year, volumes actually grew 5 percent in the first quarter.
"While we thought that some customers would use other companies, disagreements over pricing have led many to tell us they want us to deliver for them," Senior Managing Executive Officer Kenichi Shibasaki told a news briefing.
He said that Yamato did not want to refuse business even if it was driving down profits but would continue its efforts to renegotiate contracts with retailers.
Japan's lack of workers was cited as one of the domestic firms' biggest worries over the next three years, a Reuters Corporate Survey showed, with 26 percent of companies worried that they would not be able secure sufficient staff.
The country's working-age population has dropped 11 percent since its peak in 1995 to 77.2 million in 2015 and is projected to tumble to 45.2 million by 2065.
($1 = 110.5700 yen) (Reporting by Sam Nussey; Editing by Edwina Gibbs)