STOCKHOLM (Reuters) - Mobile payment solutions business iZettle's growth strategy is continuing apace, it said on Thursday, citing an 81 percent leap in revenue last year.

One of Europe's fastest-growing tech start-ups, iZettle is among a group of fledgling fintech businesses taking on traditional banks.

The revenue increase came at a cost, however, with its operating loss widening because of heavy spending to attract customers.

"The numbers are according to plan as we're fully focused on growth and are investing in expanding our product offering and customer acquisition," iZettle Chief Marketing Officer Johan Bendz told Reuters.

"We're growing revenue twice as fast as costs, so we're heading in the right direction."

The company reported 2015 revenue of 345 million Swedish crowns ($40 million), against 190 million crowns the previous year. The operating loss increased slightly to 258 million crowns, against 228 million crowns in 2014.

Established in 2010, iZettle offers small businesses and individuals a way to take payments using mini credit card readers that turn smartphones or tablets into cash registers. Last year it added France as a new market and launched new products such as iZettle Advance, a loan service for small businesses.

The company employs 275 people and is present in 12 markets in Europe and Latin America.

Its rival Square Inc, a U.S. mobile payments company that also offers a credit card reader and is run by Twitter Inc Chief Executive Jack Dorsey, increased revenue by about 50 percent to $1.3 billion in 2015.

Square Inc went public last year and is trading flat to its IPO price. It reported a quarterly loss in May as costs surged.

Smaller Swedish rival iZettle has raised 140 million euros ($155 million) of funding to date from investors including Intel Capital, Northzone, American Express, Index Ventures and Banco Santander.

($1 = 8.5452 Swedish crowns)

($1 = 0.9025 euros)

(Reporting by Mia Shanley; Editing by David Goodman)