LONDON — Alarmed by the suspected presence of hundreds of British jihadists among Sunni militants in Syria and Iraq, Britain increased its assessment of the terrorism threat on its own soil on Friday and said new laws would be introduced to counter what Prime Minister David Cameron called “a greater threat to our security than we have seen before.”

The tone of the warnings recalled the days after July 7, 2005, when four suicide bombers killed 52 travelers in the London transit system. The show of concern, moreover, seemed intended to bolster the government’s case for contentious new legislation to control terrorism suspects.

The terrorism threat level — an official designation assessing the likelihood of an attack — had long been considered to be “substantial,” meaning that an attack was “highly likely.” The new level — “severe” — indicates that “a terrorist attack is highly likely, although there is no intelligence to suggest that one is imminent,” Theresa May, the home secretary, said in a statement.

The new level is one below “critical,” which signifies that an attack is expected imminently.

The assessment is made by a panel of security experts and officials, independent of government officials. Several European nations have voiced fears that their citizens who have joined the Sunni militant Islamic State in Iraq and Syria, will return home trained and equipped for terrorist acts. But the British response seemed to be among the most drastic.