The foreign ministers of the three countries immediately hailed the capping of their unprecedented diplomatic effort here as an important step forward in defusing mounting tensions that have raised fears of yet another Middle East conflict.
"This is a very important day," France's Dominique de Villepin said after the ministers emerged from several hours of hard bargaining with top Iranian officials just 10 days before the expiry of an ultimatum set by the International Atomic Energy Agency (IAEA) for Iran to come clean.
"We were facing a major issue. Proliferation is a major challenge to the world, and today we found a solution to the pending issues," he said.
For his part, Germany's Joschka Fischer said the deal would help "stabilise the region". Britain's Jack Straw, making his fifth visit to Tehran in two years, said the deal was "an important step forward".
According to a joint declaration, Iran pledged to show "full transparency" to the IAEA, reiterated its commitment to the nuclear Non-Proliferation Treaty (NPT) and vowed atomic weapons had "no place" in its defence doctrine.
"The Iran goverment has decided to engage in full cooperation with the IAEA ... and clarify and correct any possible failures," the declaration said.
It also said the "Iranian government has decided to sign the additional protocol" to the NPT. This would allow the IAEA to carry out surprise visits to suspect facilities -- a key tool if Iran can ever be given the nuclear all-clear by the Vienna-based body.
Iran also "decided voluntarily to suspend all uranium enrichment and reprocessing activities", bowing to another key IAEA demand.
Hassan Rowhani, the secretary of Iran's powerful Supreme Council of National Security, said officials here would fix a date to sign the protocol, but cautioned that Iran reserved the right to resume enrichment if it were deemed necessary.
In return, Britain, France and Germany "recognise the right of Iran to the peaceful use of nuclear energy". They pledged that the additional protocol "is no way intended to undermine the sovereignty, national dignity or national security" of Iran.
Iranian officials fear intrusive inspections could represent a violation of national sovereignty.
If Iran fully implements its pledges, the Islamic republic "could expect easier access to modern technology and supplies in a range of areas", the European trio pledged.
Diplomats said that opened up the possibility of Iran receiving nuclear fuel from overseas, therefore keeping the sensitive fuel cycle and the risk of its its misuse outside the country.
The flying visit of the three ministers was a climax to months of intense and secretive diplomacy by the three countries.
It comes amid mounting tensions between Tehran and Washington as well as speculation that the United States or Israel could be considering pre-emptive military strikes against Iranian nuclear facilities.
Last month, the IAEA gave Iran until October 31 to turn over a raft of information on its nuclear programme after alarm bells were setting ringing following the discovery by its inspectors of traces of highly enriched uranium at two sites.
It says the traces came into the country on equipment bought on the black market, but has been understandably cautious on revealing the source of imported equipment given the imposition of US sanctions.
A failure to meet the conditions before the October 31 deadline could see Iran being declared in breach of the NPT, and the dossier being forwarded to the UN Security Council.
But diplomats say even Washington recognises this is not a satisfactory option, given that it cannot be assured of winning a consensus there because of Russia's multi-million dollar construction of a power plant in southern Iran.
The deal also comes despite voices in Iran's powerful hardline camp, which pulls many of the real strings of power, suggesting Tehran follow the example of North Korea and pull out of the NPT altogether.