Television advertising could be entering a period of long-term decline similar to the downturn experienced by the global newspaper industry, a leading media buying agency warned yesterday.

ZenithOptimedia, a group that books advertising space on behalf of companies, joined the ranks of pessimists predicting hard times ahead for the 30-second advertising slot. TV's reported demise is attributed to the advent of personal video recorders, which allow viewers to skip ads while watching their favourite shows, and the rise of the internet.

Zenith yesterday trimmed its growth forecasts for global advertising spending to 4.7%, creating a total of $404bn (£230.3bn), down from a projected increase of 5.4%. Its prediction for 2006 dropped to 6.1% growth, against the previous estimate of 6.5%.

Zenith took $3.6bn out of traditional media in the US, led by TV, and added $1.2bn to the internet. Its revised prediction for web advertising will see internet spending rise 8% to $16.4bn.

The media buyer warned that TV, a staple of most companies' advertising spending with a 37% share of the global advertising market, "may now be beginning a long newspaper-like decline".

According to yesterday's forecast, global spending on TV is expected to fall by $2.3bn to $148.2bn. The impact on the TV market is almost entirely due to the US and Japan, which are expected to reduce their spending by $1bn each. Zenith blames the declines in both countries on redeployment of advertising budgets, with Japan in particular seeing migration of TV spending to the internet.

There are signs that TV spending has peaked in two of its most mature markets, the US and the UK, according to the report. This could be good news for the outdoor industry in the US and UK, Zenith adds, because billboards can be an effective substitute for TV. Both reach large audiences, with outdoor gaining in favour because of its low price relative to TV.