Prime Minister Theresa May has vowed that Islamist terrorists "will not prevail" in the wake of Tuesday's murder of a Catholic priest in northern France.

The Islamic State terror group has claimed responsibility for the killing, describing the two knifemen who slit the throat of 85-year-old Father Jacques Hamel and seriously injured an 86-year-old parishioner as its "soldiers".

French President Francois Hollande - who met faith leaders and spoke with Pope Francis following the attack - said his country was at "war" with IS, adding: "To attack a church, to kill a priest, is to profane the Republic."

Speaking during a visit to Italy, Mrs May called on European states to step up intelligence-sharing, which she said was "one of the best ways in which we can work together to ensure that we deal with this threat, to protect our citizens, but also to ensure that the terrorists do not win".

And she added: "They are trying to attack our values. They are attacking our way of life. They will not prevail."

Mrs May described Father Hamel's murder as "yet another brutal reminder of the threat that we all face".

"Following on from the atrocities in Nice and Germany, it reinforces the need for action both in Europe and on the wider global stage," she said.

"In Europe, we must increase further our intelligence co-operation and share vital information swiftly and effectively, enabling us to better protect ourselves from these terrorists who seek to destabilise us."

It emerged that one of the two attackers shot dead by police in Normandy was wearing an electronic surveillance tag at the time of the attack, having been released from prison where he was being held after twice attempting to travel to Syria.

France's anti-terrorism prosecutor Francois Molins said 19-year-old Adel Kermiche's tag was deactivated for a few hours every morning, and the attack took place while it was not operating.

Kermiche and his accomplice took five hostages at the church in Saint-Etienne-du-Rouvray after bursting in during morning mass. One had three knives and a fake explosives belt, while the other carried a kitchen timer wrapped in aluminium foil and had fake explosives in his backpack, said Mr Molins.

They used nuns as "human shields" as police tried to end the hour-long siege by entering the church.

Mr Molins said the attackers, who claimed allegiance to Islamic State, cried "Allahu Akbar" - God is great - during the attack on the priest.

He added that a minor, believed to be a 16-year-old younger brother of somebody wanted by police for trying to go to Syria or Iraq in 2015, had been detained in connection with the investigation.

A nun, identified as Sister Danielle, described how Father Hamel was forced to kneel on the floor before his throat was cut.

She told BFM television: "They forced him to his knees. He wanted to defend himself. And that's when the tragedy happened.

"They recorded themselves. They did a sort of sermon around the altar, in Arabic. It's a horror."

The National Police Chiefs' Council has urged Britain's Christian community to be alert in the wake of the attack.

Deputy Assistant Commissioner Neil Basu said: "There is no specific intelligence relating to attacks against the Christian community in the UK. However, as we have seen, Daesh and other terrorist groups have targeted Christian as well as Jewish and other faith groups in the West and beyond.

"Following recent events in France, we are reiterating our protective security advice to Christian places of worship and have circulated specific advice today. We are also taking this opportunity to remind them to review their security arrangements as a precaution."

The advice came after Home Secretary Amber Rudd made a prearranged announcement of new measures to combat hate crime, including a £2.4 million fund to pay for extra security at places of worship.