JERUSALEM, March 9 (Reuters) - Israel's third-largest bank, Mizrahi Tefahot, reported a 15% jump in fourth-quarter profit that was boosted by the acquisition of smaller rival Union Bank.

Mizrahi said on Tuesday it earned 506 million shekels ($152 million) in the October-December period, up from 440 million a year earlier. Without the impact of the Union Bank acquisition quarterly profit would have slipped 0.2% to 439 million shekels.

Mizrahi, Israel's largest mortgage lender, said it has seen "a clear trend of customers returning to regular payments," after it had allowed some payment deferrals due to the COVID-19 pandemic.

Less than 2% of the bank's mortgage portfolio remains under full deferral, the bank said, a drop from 45.6 billion shekels last March to 3 billion shekels by the end of January. It expects the remainder of deferments to end in June.

Net interest income before credit losses rose to 1.69 billion shekels from 1.35 billion shekels a year earlier, while the credit loss provision slipped to 118 million shekels from 119 million. Mizrahi's Tier 1 ratio of capital to risk components, a key measure of financial strength, slipped to 10.04% in the quarter from 10.14% a year earlier. ($1 = 3.3367 shekels) (Reporting by Ari Rabinovitch Editing by Steven Scheer)