According to exclusive data from Nielsen Media Research that was commissioned by Marketing, the COI's media spend rose by almost 20% in 2008 to £178.1m.
The increase puts the government within spitting distance of P&G, which narrowly retained the title of the UK's biggest advertiser despite cutting spend to £178.7m.
The figures are likely to spark controversy, particularly in view of the broader market decline of almost 5% as the recession takes its toll on ad budgets.
The COI accepts only its own audited figures, which are published in its annual review each summer, but nonetheless defended its level of adspend.
A spokesman said that, as government policies and initiatives affect everyone in the country, it is vital they are communicated effectively. 'In 2007/8, campaigns tackled important issues such as giving up smoking, army recruitment, road safety and climate change. These are of huge social benefit and help gen-erate overall savings to the public purse,' he said.
The only media channel not to benefit from the COI's surge in spending was the increasingly beleaguered outdoor advertising industry.
The COI cut its investment in outdoor activity by 8.5%, while P&G cut its investment in the medium by almost 44% year on year.
Other big names also cut their adspend. The budgets of British Airways (-29%), Toyota (-31%) and John Lewis (-32%) were among the other casualties of the recession.