The Advertising Association/Warc report said the downgrade reflected a "more cautious outlook for UK GDP and concerns over the Eurozone in 2015".
The forecast for 2014 was also adjusted down from 6.4 per cent growth to 5.8 per cent.
Despite the downgrades, 2014 and 2015 are set to be the strongest consecutive years of growth for UK adspend so far this century.
The report, which is compiled from data provided by Warc and by industry trade bodies, found that UK adspend grew 4.2 per cent in the third quarter of 2014, reaching £4.307 billion.
This is similar to the growth seen in the first quarter and the second half of 2013, but slower than the 8.5 per cent growth seen in the second quarter, which benefited from the football World Cup.
Tim Lefroy, the chief executive at the AA, said: "Two years' growth at twice the rate of the economy is hardly a recipe for pessimism but adspend isn’t immune to pressures on other sectors.
"The underlying story, however, is of a sector continuing to feed growth, jobs and opportunity."
Richard Oliver, the head of Magna Global UK, said he "broadly agrees" with the growth predictions for the year.
He said: "I think it’s clearly great news to have two years of consecutive growth at such a decent level.
"In year where in terms of TV you’ve got the Rugby World Cup [instead of a major football tournament] and the election, which always puts a dampener on spend, it’s fantastic news.
"I think what we need to see a continued growth and output across all media to justify the increase in spend from advertisers."
In terms of particular channels, TV adspend is expected to rise by 6 per cent for 2014, radio is to see an increase of 8.9 per cent – its best performance since 2000, out of home ads are predicted to rise by 2.1 per cent and cinema adspend is to see overall growth of 2.7 per cent.
However print media channels are expected to see a decline in 2014, the AA/Warc said. Adspend for national newspapers is expected to decline by 4.3 per cent, and regional papers are to see 3.7 per cent drop.
The AA will release full-year figures for 2014 in May.