Prime Minister David Cameron has called for a nationwide minute of silence on 3 July at noon in memory of those killed in the Tunisia beach attacks.
Members of Parliament observed a minute's silence themselves in honour of the victims before the PM announced the plans, which will take place one week on from the attack in Sousse.
Some 30 Britons are believed to have been killed in the shooting. Three Irish people, one Belgian, one German and a number of Tunisians are also believed to be among the dead
Speaking in the House of Commons the PM described the attack as "brutal and sickening" and said Britain faced an "existential threat" from the Islamic State (Isis), calling the battle against the terrorist group "the struggle of our generation".
Home secretary Theresa May has laid flowers on the beach where the attack took place. She also held talks with her Tunisian, German, French and Belgian counterparts to discuss the threat from IS.
Authorities have delayed naming the victims of the atrocity because many were not carrying identification when they went to the beach. It means families of Britons caught up in the incident face an anxious wait to see if their loved ones were killed.
May said "every effort" was being made to make sure families received 100% accurate information.
The Met Police has already deployed staff to Tunisia to assist in the aftermat,h while the RAF is preparing to fly to the north African country to help evacuate casualties.