* Sees low-single digit NII growth in 2024 after 2023 rise of 24%

* Q4 NII up 12.4% year-on-year but down 2.5% q/q

* Q4 net profit doubles, beats forecast, but falls vs Q3

* Books record full-year net profit of 1.33 bln euros

* Announces buyback plan of 340 mln euros

(Recasts lead to lead on net interest income forecast this year, gives details on customer spread in paragraph 5, details on profits and ROTE in paragraphs 9, 10-11)

MADRID, Feb 1 (Reuters) - Spanish bank Sabadell on Thursday struck a cautious tone about the outlook for net interest income growth in 2024 after lending revenue fell in the fourth quarter of 2023 from the previous three-month period.

The bank said it expected low-single-digit lending income growth in 2024, down from a 24.3% rise in 2023, which came close to growth of about 25% the bank had targeted for the year.

Spanish banks are mainly retail lenders and have benefited from higher return on loans, overwhelmingly with floating rates, while keeping a lid on rates paid to savers.

However, central bankers have been warning that deposit costs would gradually pick up and hit margins.

Sabadell's results showed its customer spread, the difference between the yield on loans and the cost of deposits, remained unchanged at 2.99% in the fourth quarter compared to the previous quarter due to higher deposit costs at its British unit TSB.

Net interest income (NII), which is earnings on loans minus deposit costs, rose 12.4% year-on-year in the quarter to 1.21 billion euros ($1.30 billion), slightly below the 1.24 billion euros expected by analysts.

But investors are already watching closely to see if the boost to lenders from higher rates has peaked.

In the fourth quarter, Sabadell's NII fell 2.5% compared to the third quarter following a Euribor 12-month benchmark rates decline.

Net profit more than doubled in the fourth quarter to 304 million euros from the same period in 2022, above forecasts of 276 million euros, but was 34.5% below the third quarter due one-off charges and lower borrowing costs in the final quarter.

For the whole of 2023, the bank's net profit rose 55% to a record 1.33 billion euros on lower provisions.

This helped improved its return-on-tangible equity ratio (ROTE), a measure of profitability, by 373 basis points in the year to 11.5% at the end of 2023. For 2024, the lender set a ROTE target of above 11.5%.

The bank also said that its board approved a new share buyback programme of 340 million euros and a final dividend of 0.03 euros per share against 2023 earnings, both subject to shareholder approval. ($1 = 0.9273 euros) (Reporting by Jesús Aguado; Editing by Inti Landauro and Jamie Freed)