Global advertising expenditure is predicted to reach $581 billion at the end of 2018, with most of the growth coming from sponsored content in search engines and social media ads, according to industry forecaster Zenith. Zenith, owned by France's Publicis, raised its forecast for global advertising spending growth to 4.5 percent from 4.1 percent and predicted two-thirds of that growth will come through paid search and social media ads. In recent times, advertising companies including market leader WPP Plc have been hit by clients switching to using online platforms like Google and Facebook to reach consumers.

Paid search, which inserts sponsored content into search engine results, will grow $22 billion in 2018 whereas social media ad spending is expected to grow by $28 billion, making it the largest contributor to expenditure growth, Zenith said. Indicating that the UK ad market will continue to grow, Zenith said that economic growth in Britain has picked up this year and demand from advertisers has been stronger than expected.

"The UK is expected to remain the fourth country in the top ten ad markets in 2020," Zenith added. It raised its forecast for advertising spending growth in the UK to 2.4 percent from 0.7 percent for 2018. Zenith left its 2019 global advertising spending forecast unchanged at 4.2 percent but cut 2020 growth expectations to 4.2 percent from 4.3 percent.