BERLIN – The threat of a terrorist attack on European soil by Islamic radicals has increased substantially in recent months, reaching its highest levels since the London transit attacks of July 2005, according to European counterterrorism officials. Adding to the anxiety: fresh threats against Britain and France delivered Wednesday by al-Qaeda’s deputy leader. In a new videotape, Ayman al-Zawahiri, the second-in-command to al-Qaeda chief Osama bin Laden, singled out Britain as a historical enemy of Muslims, blaming it for the creation of the state of Israel and the downfall of the Ottoman Empire in the early 20th century.