“To our shock and grief we must add another word: resolve. Unshakeable resolve. We will stand up for our way of life,” Cameron wrote in British newspaper the Daily Telegraph calling for “a response at home and abroad”.

“We must be stronger at standing up for our values — of peace, democracy, tolerance, freedom,” Cameron wrote.

“We must be more intolerant of intolerance — rejecting anyone whose views condone the Islamist extremist narrative and create the conditions for it to flourish.”

The number of British victims may rise to more than 30 from the official toll of 15, the BBC reported Monday.

Three Irish people, a Belgian, a Portuguese and one German were also killed when a gunman opened fire in the beach resort of Sousse on Friday.

Cameron called for efforts against online propaganda by groups like ISIS, which claimed responsibility for the attack, but also said there was a need to strengthen political institutions abroad.

“We must also deal with it at its source, in places like Syria, Iraq and Libya, from where ISIL is peddling and plotting its death cult,” Cameron wrote, using another term for IS.

Carrying out such an attack during the holy month of Ramadan was an “insult to all Muslims worldwide”, Cameron wrote.

“This is not the war between Islam and the West that ISIL want people (to) believe. It’s between the extremists who want hatred to flourish and the rest of the world who want freedom to prosper.”

Cameron described the attack as another example of the “evil” seen in attacks in Iraq, Kenya, magazine offices in Paris and schools in Pakistan.

Flags were to be flown at half mast over Cameron’s Downing Street office on Monday in sympathy with the victims and their families.

It was the worst loss of British life in a terror attack since 52 people died in suicide bombings on the London transport system on July 7, 2005.