A British PhD student has been sentenced to life in prison in the United Arab Emirates (UAE) for spying.
Matthew Hedges, 31, of Durham University, has said he is innocent, and that he had been researching the country's security strategy.
Prosecutors say he admitted the charges in an Abu Dhabi court, which found him guilty of "spying for or on behalf of" the UK government.
The PM said the UK was urgently seeking talks with the Emirati government.
Theresa May said Foreign Secretary Jeremy Hunt was "seeking a call with Foreign Minister Abdullah bin Zayed".
Mr Hunt said he was "deeply shocked and disappointed" by the verdict.
He said he had raised the case with the "highest levels of the UAE government", adding the verdict was "not what we expect from a friend and trusted partner of the United Kingdom, and runs contrary to earlier assurances".
In a statement, Hedges' family said that during the first six weeks of his detention he was interrogated without a lawyer and consular access was unavailable.
During this time he was made to sign a document in Arabic which transpired to be a confession, they said.
"Matthew does not speak or read Arabic," the family added.
Hedges' wife Daniela Tejada, who was present during the brief hearing earlier, said she was in "complete shock".
She said: "Matthew is innocent. The Foreign Office know this and have made it clear to the UAE authorities that Matthew is not a spy for them.
"This whole case has been handled appallingly from the very beginning with no-one taking Matthew's case seriously."
She said the British government "must take a stand now" and the UAE authorities "should feel ashamed for such an obvious injustice".
Ms Tejada said her husband shook his head as the verdict was delivered, adding: "I am very scared for Matt.
"I don't know where they are taking him or what will happen now. Our nightmare has gotten even worse."
At prime minister's questions, Tory MP Crispin Blunt told Mrs May she should make clear to the UAE that "if he is not released, I don't see why we should be committed to their defence".
BBC diplomatic correspondent
I was confidently expecting that we would hear some good news today because all the indications through private channels, and things I understood the family had been hearing, that the Foreign Office had been hearing, that MPs had been hearing, was that the UAE was frankly a little embarrassed about this case and wanted it over with.
So no-one was expecting the sentence of life imprisonment that has absolutely stunned everyone.
According to Abu Dhabi newspaper The National, a life sentence means a maximum of 25 years in jail, after which Hedges would be deported.
Hedges, who is also liable for the costs of the case, has 30 days to appeal during which time he will be held in custody, the paper reports.
Attorney General Dr Hamad Saif Al Shamsi confirmed the sentence was not final and said Hedges had the right to appeal before the Federal Supreme Court for retrial.
He said: ''The defendant confessed in detail to his crimes during investigations whereby he was accorded his full rights and assurances as per the UAE constitution and state laws to fair and transparent trial."
Dr Al Shamsi said Hedges had appeared in court with British embassy representatives.
Meanwhile, Mr Hunt urged the UAE to reconsider the case.
"Our consular officials have been in close contact with Matthew Hedges and his family," he said.
"We will continue to do everything possible to support him."
Hedges is said to be in a poor state of mental health, the BBC understands, and Ms Tejada has previously criticised the lack of treatment he has received in prison.
His family allege his physical and mental health "seriously deteriorated" during solitary confinement and he was fed a "cocktail of medication" by guards that caused him to vomit on a daily basis.
Vice-Chancellor of Durham University, Professor Stuart Corbridge, said the conditions Hedges was being held in "breached his human rights".
"[And] this judgement has been delivered in the absence of anything resembling due process or a fair trial," he said.
"There has been no information given on what basis Matt was handed this sentence and no reason to believe that Matt was conducting anything other than legitimate academic research.
"We are committed to doing what we can to get Matt home safely and swiftly."
What are the links between the UK and UAE?
By BBC Reality Check
More than 5,000 UK companies do business in the UAE - it's the UK's largest export market in the Middle East and the 13th biggest in the world.
The UK exported a total of £9.8bn of goods and services in 2016.
The top UK goods exported to the UAE last year were:
- machinery and mechanical appliances
- electrical equipment
- precious stones and metals
- motor vehicles
- optical, photographic and medical equipment
The UK imported £4.8bn worth of goods and services from the UAE in 2016.
There are about 120,000 UK nationals living in the UAE, according to the UK government.
British educational institutions have a presence too, with 11 UK universities represented.
The UK also has important military ties with the UAE and in 2012 announced a defence partnership with the country.
Last year a BBC investigation uncovered sales of surveillance equipment between a UK company and countries in the region, including the UAE.