Foreign Secretary Jeremy Hunt said there was a "small window" to save the Iran nuclear deal, as he launched a fresh bid to ease tensions in the Gulf.
Mr Hunt has been meeting EU foreign ministers to raise concerns about Iran breaching some of its commitments.
The deal, which involves Iran limiting nuclear activities in return for the easing of economic sanctions, is under pressure after the US withdrew in 2018.
The summit comes amid heightened tensions between the UK and Iran.
The UK seized an Iranian oil tanker earlier this month and, in response, Iran threatened to detain a British oil tanker.
However, Mr Hunt said on Saturday the tanker, Grace 1, could be released if the UK is guaranteed the oil it was carrying is not bound for Syria.
Speaking on his way into the meeting in Brussels, Mr Hunt said the Iran nuclear deal was not dead "yet", but he warned that if Iran acquired nuclear weapons it would become "a very, very toxic and dangerous situation".
He added: "Iran is still a good year away from developing a nuclear weapon. We think there is still a closing, but small, window to keep the deal alive."
Mr Hunt said he would work with the European partners of the deal - France and Germany - to find a way to "preserve" the agreement.
1/2 Important discussions with EU colleagues including @JY_LeDrian and @ministerBlok about Iran: we support nuclear deal but there can be no ‘partial’ compliance. You are either on path to a nuclearised Middle East or not... pic.twitter.com/XMlKixjM3f— Jeremy Hunt (@Jeremy_Hunt) July 15, 2019
In a joint statement issued ahead of the meeting, Britain, France and Germany reiterated their support for the deal.
But they said they were "deeply troubled" by recent events in the Gulf and "concerned" over US-Iran relations.
The statement added that all stakeholders should "pause and consider the possible consequences of their actions".
Earlier this month Iran said it would be breaching the deal by breaking a limit set on uranium enrichment.
Why does saving the Iran nuclear deal matter?
Even if Iran does not actually build a nuclear warhead, it only has to reach the point at which it COULD produce one for its nervous neighbours to decide this is too much of a risk.
They will want their own one too - as a deterrent.
Statements from Saudi Arabia, Iran's Middle East rival, have made it clear the country would not accept a nuclear-armed Iran.
So then, as stated by diplomats in Brussels, we are into a nuclear arms race.
What does that mean in practice?
In all probability Saudi Arabia, Turkey and Egypt would all want to become nuclear weapons powers, with the potential to obliterate entire cities.
This would be in a part of the world that has seen almost continuous conflict in places for the last 71 years.
Finally, there is the risk that should Iran ever go nuclear some fear it could pass on a warhead to a non-state militia like Hezbollah.
This is why the Iran nuclear deal matters, even if you don't live anywhere near the Gulf.
What is the Iran nuclear deal?
In 2015, Iran signed up to a long-term deal - called the Joint Comprehensive Plan of Action - after the international community expressed concerns they were trying to develop nuclear weapons.
Iran, which insisted its nuclear programme was entirely peaceful, agreed to limit some nuclear activities and allow in international inspectors.
In return, the other signatories - UK, France, Germany, China and Russia - agreed to lift some of the crippling economic sanctions placed on Iran.
Speaking ahead of Monday's meeting, Mr Hunt said: "The Middle East is already one of the most unstable regions in the world, but if the different parties were armed with nuclear weapons it would represent an existential threat to mankind."
Why is the deal in trouble?
In 2018, Donald Trump said he would unilaterally withdraw the US from the agreement which was signed under the Barack Obama administration.
The other parties criticised Mr Trump's decision and said they remained committed to the deal.
The Mail on Sunday published a leaked memo from the UK's ambassador in Washington which said Mr Trump abandoned the nuclear deal to spite Mr Obama.
Earlier this month, the International Atomic Energy Agency confirmed that Iran had breached the deal's cap on stockpiling of low-enriched uranium.
Iran said it was responding to sanctions reinstated by the US after Mr Trump abandoned the deal. Last week it confirmed it will break another of the limits imposed by the deal.
Deputy foreign minister Abbas Araqchi said Iran still wanted to salvage the deal but blamed European countries for failing to live up to their own commitments.
What about the tanker tensions?
Tensions between the UK and Iran flared up earlier this month when Royal Marines seized an Iranian tanker which was suspected of breaking EU sanctions.
The UK suspected Grace 1, detained on 4 July near Gibraltar, was carrying oil bound for Syria.
Iran denied it was headed for Syria and claimed the seizure of the ship was "piracy".
In a phone call with Iranian foreign minister Mohammad Javad Zarif on Saturday, Mr Hunt sought to reassure his counterpart that the detainment "was nothing to do with the oil being Iranian".
Mr Zarif also wanted to resolve the issue and was "not seeking to escalate" the situation, Mr Hunt said.
In response to the seizure, Iran threatened to seize a British oil tanker.
On 9 July, the UK raised the threat to British shipping in Iranian waters in the Gulf to "critical" - the highest level.
A day later, Iranian boats attempted to impede a British oil tanker in the region, before being warned off by a Royal Navy ship, according to the MoD.
Iran denied any attempted seizure, with Mr Zarif quoted as saying the UK made the claims "for creating tension".
How will the US react to EU statement?
Analysis by BBC diplomatic correspondent James Robbins
Britain, France and Germany have made a strongly worded intervention to try to prevent the crisis in relations with Iran deteriorating into something far worse: armed conflict.
The focus of their joint statement is on the need to try to preserve the 2015 nuclear deal which President Trump abandoned in favour of increasing sanctions against Iran.
It's a deal which Iran now says it is breaching because it is not delivering the promised economic benefits.
But Theresa May, Emanuel Macron and Angela Merkel have said they are concerned by the risk that the nuclear agreement "further unravels" under the strain of US sanctions and Iran's decision to no longer implement key parts of the deal.
They went on to urge all countries "to pause and consider the possible consequences of their actions."
That's likely to infuriate the White House, seeming to equate aspects of United States hard-line policies with those of Iran.