Theresa May has told Egypt's president that the UK "stands ready to help in any way possible" following the terrorist atrocity at a mosque which killed more than 300 people.
The Prime Minister offered her condolences to Abdel-Fattah el-Sissi after the outrage at Bir al-Abd on the Sinai Peninsula on Friday.
Downing Street said the two leaders agreed that international co-operation was needed to tackle the problem of terrorism.
A Downing Street spokesman said: "The Prime Minister offered her personal condolences and the condolences of the British people for the barbaric attack that took place at al-Rawdah mosque in north Sinai yesterday."
The attack was a "terrible example of the threat Egypt faces" and "they agreed that terrorism was an international problem and that it was important for countries to work closely together and share expertise to counter the growing threat of terrorism around the world".
"They welcomed continued bilateral co-operation in this area."
Egypt's chief prosecutor Nabil Sadeq said the attack was carried out by 25 to 30 militants who arrived at the mosque in the small town of Bir al-Abd in five all-terrain vehicles.
They stationed themselves at the mosque's main door and 12 windows before opening fire on worshippers inside.
The mosque is frequented by Sufi Muslims, a mystic school of Islam that militants - including the local affiliate of the Islamic State group - consider heretic.