World leaders have not yet made any deal with Iran over its controversial nuclear programme, with "important gaps" still to be closed at the tense negotiations in Geneva.
Foreign Secretary William Hague tweeted from the Swiss city:
And he shared a picture of the historic meeting, with Iranian Foreign Minister, Javad Zarif, German Foreign Minister Guido Westerwelle and Baroness Ashton, EU foreign policy chief. US Secretary of State John Kerry was nowhere in sight.
Hague decided to attend in the hope the presence of the E3+3 negotiating group - the UK, France, Germany, Russia, China and the US - might help the process move towards a successful conclusion.
"It is for Iran to be able to convince the international community that it is able to address the deep concerns that the international community has," Downing Street said.
"We have always said we wanted to find a diplomatic solution. That is why we have had these E3+3 talks."
Among sanctions it is believed could be lifted are a freeze on up to £31 billion in overseas accounts and restrictions on petrochemicals, gold and other precious metals with those on oil exports and the financial sector expected to remain in place pending more detailed negotiations on a full deal.
Hague said the atmosphere at the talks was "completely different" from a few months ago but stressed that there was no certainty of any agreement being reached by the end of the day.
"There are still important issues to resolve. We are going to have to give a lot of time and attention to those issues during the course of today and there is no fixed time for us to reach a conclusion," he told reporters.
"It's certainly not possible to say that we can be sure there will be a deal at the end of today. And if there isn't of course then we must continue to apply ourselves in the coming weeks, building on the progress that has been made.
"The atmosphere at these negotiations...is completely different from the atmosphere of a few months ago."
Kerry sat down with his Iranian counterpart and Ashton for five hours yesterday.
Washington and Tehran both suggested progress had been made but that there was more work to do, with Kerry speaking of "important gaps" still to be closed.
French and German foreign ministers are also at the meeting which is seeking a deal to ease sanctions in return for guarantees Iran's nuclear ambitions are civil not military.
Russian foreign minister Sergey Lavrov and a Chinese deputy foreign minister are expected to join them.
The attempted deal - reported to allow Iran to continue enriching uranium so long as it is only to the level required for nuclear power and not weapons - has been criticised by Israel.
Prime minister Benjamin Netanyahu said any such agreement was a "bad deal" as it would not stop Iran resuming military-related activities.
And Saudi Arabia, perhaps Iran's most bitter enemy, has made clear to Washington that it does not like the idea of any easing of relations.
Any breakthrough would be the first after nearly 10 years of efforts and potentially pave the way towards a more comprehensive agreement to prevent Tehran producing nuclear arms.
Iran has always denied it has any ambition to do so, insisting its nuclear programme is entirely for the purpose of energy, medical treatments and research.
A senior US State Department official said Kerry was committed to doing "anything he can" to overcome the chasm with the Islamic Republic, Reuters reported.
Prime Minister David Cameron and French president Francois Hollande discussed the situation in a phone call and agreed there was a "real opportunity to make significant progress", Downing Street said.