Facebook has said that it will give advertisers more control over where their ads appear on the site after several big companies and the Government withdrew their accounts.

The networking site said that it had been caught unawares when at least six companies, including Vodafone, and the Central Office of Information (COI) announced they would no longer advertise on Facebook because their ads were appearing next to a profile for the British National Party.

"We weren't entirely prepared for it or we would have taken action to try to resolve it before it even got to the press," Owen Van Natta, Facebook's chief operating officer, said.

Mr Van Natta said that the site would address the problem by building technology which would allow companies to target their ads, and introduce "some additional controls" which would enable advertisers to avoid unwanted associations.

"I expect that most, or not all, of those advertisers are going to be coming back on board if they haven't already," Mr Van Natta said.

Last week, First Direct, Virgin Media and the AA joined Vodafone in a boycott of Facebook, and this week the COI directed its buying agency, i-level, not to purchase any ads on internet sites with user-generated content.

Virgin Media today welcomed Facebook's announcement, saying that it looked forward to discussing a solution. "The issue on these sites has been that we haven't able to be confident what type of content our ads are appearing next to, but if they can find a way around that, then we'll definitely be back on board," a spokesman said.

Vodafone said that it had not yet changed its position, but confirmed that Facebook had contacted its agency, OMD, about the changes, which would prevent ads from appearing on 'group' profiles if an advertiser wished.

The COI said that it too was "in discussions" with Facebook, but that its ads remained "pulled" for the time being.

Brand owners have been keen to target the audience that uses social networks, but the lack of control such sites have over the content users post makes them a potentially high-risk platform.

Among the groups users can join on Facebook are 'A call for Jihad', 'Boycott Zionist/Israeli goods' and 'Thumbs up for Leprosy'.

The Internet Advertising Bureau, the UK industry body, said that social networks should consider introducing a means of reassuring companies that pages were "safe", or risk more advertisers abandoning the medium.