LONDON—If the extremist group fighting in Iraq and Syria that calls itself the Islamic State isn’t stopped it will eventually seek to carry out an attack on British soil, U.K. Foreign Secretary Philip Hammond warned in an article published on Sunday.

Writing in the Sunday Times newspaper, Mr. Hammond said security services estimated some 500 British jihadis had travelled to Syria and Iraq in recent years. The likely involvement of a British citizen in the killing of American journalist James Foley was further proof of the…