FRANKFURT/MADRID/MILAN, Jan 29 (Reuters) - European banks' run of soaring profits and record shareholder payouts faces a big test this week when investors assess how fast the boost from higher interest rates is fading, and if a weak economic outlook will make life tougher.

Spain's BBVA reports fourth-quarter numbers on Tuesday, Santander on Wednesday, Deutsche Bank and BNP Paribas on Thursday and UniCredit the following Monday. Other euro zone banks and Switzerland's UBS follow.

The STOXX Europe 600 banks index hit its highest since mid-2018 this month, propelled by a recovery in profitability thanks to higher rates, record shareholder payouts and little provisioning for bad loans.

Yet as banking executives bask in the good times, investors are concerned that the tide is turning.

PAST THE PEAK

Retail-focused lenders which earn most of their money from the difference between loan income and deposit costs have benefited most from rising rates, but bigger, diversified banks such as Deutsche and BNP Paribas have also seen profits.

Investors appear jittery, however, with a small miss in fourth-quarter net interest income (NII) from Spain's Bankinter last week causing a 6% drop in its stock price and dragging down rivals' shares.

JP Morgan analysts warn that lower rates will lead to a "downgrade cycle" across the sector. After an estimated 22% net interest income (NII) jump in 2023, the bank expects European lenders to show limited NII growth this year and zero earnings growth.

UniCredit is expected to project a 4% drop in NII for 2024 according to the consensus forecast, Jefferies analysts said, although they expect the bank to beat expectations. It has excess capital - 10 billion euros ($10.9 billion) worth - that it needs to decide what to do with.

Analysts will also be listening closely to BNP Paribas' results and what management decides to do with the excess capital it still has from the sale of Bank of the West.

NOT SO BAD

Not everyone believes falling margins are such a concern.

The average 2024 European interbank lending rate should be higher in 2024 than in 2023 and interest income will only "be down a bit", said Sebastiano Pirro, chief investment officer at Algebris Investments, which holds shares in banks.

"What you are looking at for European banks is a 10-year inflection point," he said, referring to a decade of negative euro zone rates that destroyed bank profitability.

"Today banks are making more money than they can distribute and capital ratios are going up," he added.

Among other banks, Santander and BBVA are expected to report higher net profit and NII versus 2022, helped by their Spanish and Latin American businesses.

At Deutsche Bank the situation is less rosy. Analysts expect fourth-quarter net profit attributable to shareholders of around 700 million euros, down from 1.8 billion euros in 2022. That would still mark the 14th straight quarterly profit after years of losses.

WEAKER LOANS

Investors will also be watching closely to see if loan quality is worsening as higher interest rates bite.

Increased borrowing costs are yet to swell banks' non-performing loan book, with the only real stress in commercial real estate, mostly in Sweden and Germany.

Algebris' Pirro said that the clean-up of non-performing loans in recent years had been so extensive in southern Europe, and the demand for new loans so muted, that bad loans will remain small.

Still, weak economic growth forecasts will dampen loan demand across the bloc, and executives remain cautious.

Manfred Knof, Commerzbank CEO, told Reuters last week that he was bracing for another year of stagnation.

There's no sign of a wave of defaults, but "companies are reticent to invest. The investment backlog in the German economy is getting bigger every day, we are already noticing that."

INVESTMENT BANKING

The big five Wall Street banks reported fourth-quarter year-on-year falls in trading and investment banking revenues of 20% and 17%, respectively, although whole-year drops were far smaller, according to Barclays' calculations.

Analysts will want to see how Deutsche Bank, BNP Paribas and UBS, which have large investment banking businesses, are faring.

Barclays analysts reckon fourth-quarter numbers in Europe will be similar to the third-quarter, with wealth management holding up and capital markets revenues up slightly while advisory fees are down.

MERGER CHATTER

Chatter about long-delayed European bank consolidation resurfaced this month, although it was quickly talked back down.

Deutsche Bank and Commerzbank CEOs poured cold water on the prospect of a tie-up in Germany. In Italy, UniCredit boss Andrea Orcel dismissed speculation it was buying shares in a smaller rival.

But with banks flush with more cash than they have been for years, expect executives to lay out how M&A - at the right price and at the right time - remains on the table. ($1 = 0.9208 euros)

(Reporting by Tom Sims, Jesus Aguado, Valentina Za and Matthieu Rosemain; Additional reporting by Klaus Lauer and Sinead Cruise Editing by Tommy Reggiori Wilkes and Louise Heavens)