Two nuns who worked for decades at a Catholic school in California embezzled a “substantial” amount of money from tuition and other funds and used it to pay for gambling trips to Las Vegas, church officials have said.

Sisters Mary Margaret Kreuper and Lana Chang are believed to have siphoned off cash from tuition fees and donations at St James school in Torrance, near Los Angeles, for at least a decade. Neither has been charged with a crime.

Kreuper was the school’s principal and Chang taught eighth-grade students before the pair both retired this year.

A spokesman for the archdiocese of Los Angeles, Adrian Alarcon, said the alleged theft was discovered during an audit, routinely held after a principal leaves office.

The total taken from the school was still being calculated, Alarcon said, adding he could not confirm reports that it was up to $500,000 (£400,000).

In a letter to parents, Monsignor Michael Meyers described the sum as substantial. “This matter came to our attention during financial reviews in connection with the change in leadership at our school,” he said, adding that no one else was implicated.

At a meeting he is reported to have told parents that around the same time as the audit, a family requested a copy of a cheque made out to the school and staff noticed it had been deposited in a bank account that was not the school’s.

Meyers said both nuns acknowledged the theft when confronted, apologised and were cooperating with an investigation. He said they and their order, the Sisters of St Joseph of Carondelet, had promised to pay back the money.