British broadcaster ITV today forecast a sharper than expected drop in advertising revenue in the current quarter, overshadowing a strong start to the year for the home of the Downton Abbey drama.

ITV, which has rebuilt itself in recent years by selling programmes to international markets and growing its online business, said its recent weak viewing figures had also continued.

Net advertising revenue (NAR) was up 12% in the first three months of the year, ahead of the 11% it had previously forecast. 

However, the group said it now expected net advertising revenue to come in just 5% higher for the first half of the year as a whole. 

That implied a rise of 5% in April and a forecast drop of 5% in May and a fall of between 5-7% in June due to strong comparative figures in the previous year. 

The trading update coincided with a 24-hour strike by some ITV staff who are angered at the 2% pay rise they have been awarded, despite the improvement in the company's fortunes. 

The network's recent share of viewing also remained weak, down 3% in the first four months but the group flagged new programmes which had been well received, such as reality show Britain's Got Talent and drama Code of a Killer. 

It should also enjoy a lift from the Rugby World Cup which kicks off in September and which ITV will show exclusively. 

"Our digital channels are growing audience share, up 3% overall year on year, and we are firmly focused on the main channel where we expect to see improvement in the second half of the year," it said.