In spite of the explosive growth in the use of smartphones in recent years, mobile operators across Europe are struggling to improve their average revenue per user.

Competitive pressure from over-the-top players including Facebook Inc.'s WhatsApp and Microsoft Corp.'s Skype in core areas such as messaging and calling has made it even more crucial for the telecom industry to reverse years of slowing revenue streams.

But even with unprecedented demand for data, propelling three years of subscriber gains across some of the biggest operators in France, Germany, Italy, Spain and the U.K., ARPU either declined or stayed flat for the most part, according to S&P Global Market Intelligence data.

The steepest decline in earnings per user over a 36-month period was at Telefónica O2 UK Ltd., Telefónica SA's U.K. arm, where ARPU was down 18%. This contributed to an 11.3% fall in the weighted average ARPU for the U.K. over three years.

This was despite a return to growth last year in U.K. mobile service revenues, up 2.2% year-over-year to an estimated £16.7 billion, according to S&P Global Market Intelligence.

Operators across Britain also saw a downward trend in end users at the end of 2017. Ongoing competition reduced the subscriber count at BT Group's EE by 2.3% to 29.8 million, while Vodafone Group PLC's U.K. unit decreased by 4.3% to 17.6 million. Meanwhile, O2 closed 2017 with 25 million customers, down 1.9% from the prior year but flat compared to 2015.

Italy mostly bucked the trend over three years, however, delivering weighted average ARPU gains of 4.2%.

Each telco held approximately a third of the market prior to this year's entrance of French telecom operator Iliad, whose low-cost pricing strategy reportedly handed it 1 million customers in 50 days.

On an individual basis, TIM, or Telecom Italia, delivered a 3.3% rise in ARPU and a 2.5% boost in subscriber numbers. Wind Tre SpA and Vodafone Italia both took a hit to their customer bases, but either increased or maintained their ARPU during the same period.