Oxfam and WWF International, two of the world's largest and most conservative western climate change campaigning groups, have been forced to apologise profusely after an investigation found them guilty of what amounted to diplomatic terrorism: smashing Saudi Arabia's official UN nameplate and stuffing the pieces down a lavatory.

The incident, which took place in June at the last global climate talks in Bonn, followed mounting anger by NGOs at what they interpreted as continual blocking tactics by oil giant Saudi Arabia in the global talks.

On the last day of the negotiations, two activists from WWF and one from Oxfam stole the Saudi nameplate, which sits on delegates' desks to identify their origin, broke it and then took photographs of the pieces in the bowls of both the men and women's toilets of the Maritim hotel, where the talks were taking place. They then distributed photocopies with the headline: "Feeling a bit blocked?"

But what might have been dismissed as a childish end-of-term prank by passionate, over-enthusiastic campaigners became an international incident after Saudi Arabia demanded a full UN investigation and took full diplomatic advantage of the incident.

The two NGOs owned up and offered to apologise publicly, but Saudi Arabia then demanded that both organisations be banned from attending any UN climate talks for five years, a punishment which would have impacted heavily on both organisations' work. All three employees have reportedly now left the NGOs.

This week all negotiations on climate change were suspended for several hours as diplomats lined up to condemn the activists or to defend free speech.

"The severity of that heinous incident dictates that a position must be taken lest similar incidents occur in the future. Acts of intolerance from a sector that we always held in high esteem – namely civil society and NGOs – only further the objectives of fanatics," said a spokesman for Yemen.

Jonathan Pershing, lead negotiator for the US, said the action was "totally unacceptable", but stood up for free speech. "NGOs are valuable participants in the negotiations. They keep the public informed in what we are doing. We cannot undermine civil society's role in our discussions or have this incident cow or limit their voices."

The NGOs were contrite, both sending senior representatives to apologise. "The act itself was repugnant. We condemn it utterly. We failed to respond in a serious and timely manner," said Oxfam.

"I am very, very sorry. We deeply regret the offence we have caused. These actions by a WWF employee went against the repectful and democratic principles and values of our organisation. We'll ensure something like this never happens again," said president of WWF International, Yolanda Kakabadse.

The verdict, handed down by Christiana Figueres, the new executive secretary of the United Nations Framework Convention on Climate Change, was itself diplomatic. One WWF member was banned from the talks for life, and two others, one from each group, were banned for the rest of the year. In addition, WWF will only be allowed to send two people to the next round of negotiations in China in October, and Oxfam will be permitted three.

Saudi Arabia, which has been accused of slowing the talks for years, released a statement apparently drawing a line under the affair: "Listening to the very clear apologies, we are a forgiving society, and we think we would like to say we are not going to seek any further action against the two organisations. We would like to consider the case closed."