The growth was "well ahead" of the previous prediction of 6.2 per cent, and has led to an upwards revision of 0.6 percentage points to 6.2 per cent for the 2015 forecast.
It has been fuelled by "particularly strong" traditional display adspend, including outdoor and radio. Overall display adspend rose 8.5 per cent and climbed to £3.2 million, compared with the first quarter in 2014.
TV saw an 11.5 per cent rise to £1.2 billion, the highest-ever first quarter total for the sector. Radio improved by 8.2 per cent to £122 million. Out of home rose by 9.7 per cent to £229 million.
The continued rapid increase in mobile spend also played a part. It rose by 50.9 per cent in the first three months of the year to £502 million.
However, it was not a similar story in the print sector as national newsbrands saw slower digital growth of 7.4 per cent, the lowest on record. The total adspend declined 6.8 per cent in the first quarter of the year, compared with Q1 in 2014, and a 3 per cent drop is predicted for 2015.
Tim Lefroy, the chief executive at the Advertising Association, said: "Despite uncertainty in Europe and at home prior to the election, these figures come as a welcome boost.
"Adspend is growing faster here than anywhere in Europe, to the benefit of our digital economy, creative industries and UK plc."