[PARIS] Britain warned on Tuesday that stadiums and fan zones could be targeted during Euro 2016, adding to security fears over the football extravaganza in France.
The warning came a day after Ukraine announced the arrest of a suspected far-right French extremist with a massive weapons cache who was allegedly planning a string of attacks to coincide with the tournament.
The Foreign Office warned British citizens there was a "high threat from terrorism" during the month-long championship, though Prime Minister David Cameron's spokeswoman said the advice was not a response to the arrest.
"During Euro 2016, stadiums, fan zones, venues broadcasting the tournament and transport hubs and links represent potential targets for terrorist attacks," the Foreign Office said.
French Foreign Minister Jean-Marc Ayrault, on a visit to London, said he had reassured his British counterpart Philip Hammond "and reminded him of the excellent cooperation between our intelligence services and police".
The US State Department gave a similar warning last week, adding that "unaffiliated entertainment venues broadcasting the tournament in France and across Europe" were also potential targets.
Just three days before the opening match between France and Romania on Friday, France faces the threat of continued disruption to rail travel due to strikes that have paralysed parts of the network for a week.
Unions voted to continue the train strikes for an eighth day on Wednesday despite receiving an improved offer on pay and conditions from state-run operator SNCF in all-night talks.
"There comes a time when you need to know when to stop a strike," President Francois Hollande said.
The CGT union, which has led three months of protests and strikes against controversial government labour reforms, replied sardonically: "There comes a time to end a strike, but only when we have got satisfaction."
The rail strike was limited in scope on Tuesday, but even if trains return to normal the threat of a walkout by Air France pilots still hangs over the four-yearly gathering of Europe's top football nations.
The pilots are set to ground planes for four days from Saturday over pay, just when an estimated two million foreign visitors will begin heading for France.
Hollande has warned the unions they will receive little sympathy if they disrupt the tournament.
CGT activists blocked access to two of the terminals at Paris' main airport, Charles de Gaulle, for over an hour on Tuesday in an unannounced protest to call for the labour legislation to be scrapped.
The unions have also called for a national day of demonstrations on June 14 against the labour reforms, which are designed to make it easier to hire and fire people, but which opponents say will erode job security.
With the threat of a jihadist attack already hanging over Euro 2016, the arrest of a Frenchman with an arsenal of weapons in Ukraine has raised new security fears.
Ukraine's security service SBU said Monday that the 25-year-old, identified in France as Gregoire Moutaux, was planning to attack multiple locations including a mosque, synagogue, tax offices and transport checkpoints.
But France has made no official comment on the arrest and anti-terrorist prosecutors have not been assigned to the case, suggesting French authorities do not believe there was any imminent threat to Euro 2016.
SBU chief Vasyl Grytsak said the suspect "expressed negative views about his government's approach to the immigration of foreigners into France, the spread of Islam and globalisation".
He was arrested on May 21 while trying to cross from Ukraine into Poland, in a van containing a massive weapons stash that included 125kg of TNT, Kalashnikov rifles and grenade launchers.
The man worked for an agricultural cooperative inseminating cows in eastern France and the firm's technical director Luc Voidey described him as "an exemplary employee".
Neighbours in the village of Nant-le-Petit, where the man regularly stayed with his grandfather, expressed astonishment at his arrest, saying he had never expressed political views.
France has mobilised 90,000 security personnel to guard Euro 2016.
The country is still under a state of emergency following November's suicide bombings and shootings in Paris that left 130 people dead and were claimed by the Islamic State group.