ZenithOptimedia expects global adspend to reach $544 billion in 2015, a 4.4 per cent rise year on year.
In April last year ZenithOptimedia forecasted growth of 5.8 per cent for 2015 and at the time it was the third consecutive revision upwards. Since then the forecast has been downgraded three times.
According to the report, the UK is "booming" because of the "rapid adoption" of online ads.
In the UK adspend grew approximately 8.4 per cent in 2014, and ZenithOptimedia said it expects a similar increase for 2015.
The UK is currently the fifth largest ad market at $22.6 billion, however ZenithOptimedia predicts the country will rise to fourth place above Germany by 2017 as UK adspend will increase to $26.8 billion.
Last year, the UK was also in fifth place, with adspend of $20.4 billion. In April 2014 ZenithOptimedia predicted the UK ad market would rise to $23.9 billion by 2016.
The US, China, Japan and Germany are currently above the UK in the table of top ad markets.
In terms of adspend by medium ZenithOptimedia said mobile internet would take a greater share of the market. Mobile’s share is expected to increase by 7.4 percentage points, taking 12.7 per cent share of global adspend by 2017.
ZenithOptimedia said mobile internet is growing nine times faster than desktop internet. The report forecasts that mobile advertising will grow by an average of 39.8 per cent a year, while desktop internet is expected to grow at an average of 4.6 per cent a year.
However, this growth is slower than predicted last year when ZenithOptimedia said mobile advertising will grow by an average of 50 per cent a year between 2013 and 2016. It expected desktop internet advertising to grow at an average of 8 per cent a year.
Global TV adspend is expected to take a smaller share of the market by 2017, although it remains the dominant advertising medium.
The report said it had a 39.4 per cent share of global adspend in 2014, and predicts this to be 37.3 per cent in 2017.
ZenithOptimedia forecasts TV adspend will grow by an average of 3 per cent a year through to 2017.
Steve King, the worldwide chief executive at ZenithOptimedia said: "While television will remain dominant for many years to come, advertisers are increasingly utilising online video as an invaluable complement, giving them new opportunities to communicate brand values to consumers."