A custodian at a Upper East Side church was slashed by a man who’d been banned from the Catholic facility — but still tried to use its restroom.

Frank Fernandez tried to enter Saint Catherine of Siena Church at East 68th St. and First Ave. to use the bathroom on Saturday — 15 minutes before the church’s closing — when he was turned away because of previous problems in which he was involved at the church, police sources and court documents noted.

Fernandez was already banned from the church, court documents showed.

But Fernandez refused to leave, pulling a boxcutter and slashing the 63-year-old custodian in the left arm. Cops took Fernandez into custody, charging him with burglary and assault.

At his arraignment in Manhattan Criminal Court, prosecutors requested Fernandez be held on b

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