The vast majority of 120 websites surveyed by the Advertising Standards Authority adhered to its extended advertising code before it was introduced, according to the self-regulatory body.
On 1 March, the ASA extended the remit of its advertising code of conduct (Cap Code) to cover companies' non-paid marketing online, such as advertising on their own sites and social network spaces.
In the two months prior to this date, the ASA monitored 120 websites belonging to a broad sweep of large, medium and small businesses in eight sectors including entertainment, media, leisure, travel, retail and education.
The ASA did not disclose the identity of individual sites monitored, but found six of them contained marketing communications that were in breach of the Cap, amounting to a compliance rate of 95%.
It found 22 of the 600 pages assessed included marketing communication that was clearly in breach of the Cap Code – a compliance rate of 96.3%.
Four of the six websites that included a breach of the Cap Code came from the entertainment, media and leisure sector, and all four related to the misleading omission of mandatory charges (eg booking charges) from the headline price.
Guy Parker, the chief executive of the ASA, said: "The ASA now expects all advertising online to be legal, decent, honest and truthful.
"That's good for the public and it's good for responsible businesses too. Our survey sets a high bar for future compliance rates.
"In the meantime, we'll continue to tackle advertising that breaks the rules wherever it appears, while doing all we can to help advertisers to comply."