According to its half-year financial report released this afternoon, Havas’ UK business suffered from what it called the "wait and see" attitude prompted by the June referendum on whether the UK should leave the EU.
Revenue growth at Havas in the UK, which includes agencies such as Havas London, Havas Helia and Arena Media, was also hampered by tough comparisons against a strong first half (organic growth of 5.1%) in 2015.
Havas reported other European countries experienced "very strong momentum" with organic growth of 8.7% during the first half of the year, which was driven by Germany, Italy and Portugal.
Group revenue between April and June 2016 increased 5.2% year on year on an unadjusted basis to €581m. Organic growth for the second quarter was 2.7%, slightly below 3% organic growth across the first half of the year.
Yannick Bolloré, chief executive at Havas, said: "Business continued at a highly satisfactory level with new wins including the global media account for Swarovski, the global digital, advertising and content account for five consumer healthcare categories of GSK, [Japanese pharmaceutical giant] Shionogi, and two big wins in the US.
"Our together strategy we implemented three years ago, which aims to create the most integrated and agile group in the industry to deliver better outcomes for our clients, continues to deliver strong results.
"This strong performance in the first half of the year gives us every confidence that our annual targets will be met."