LONDON – Alarmed by the suspected presence of hundreds of British jihadis among Sunni militants in Syria and Iraq, Britain increased its assessment of the terrorism threat on its own soil 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 on 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” – meant 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 designation is the second highest after “critical,” meaning that an attack is expected imminently.

“The increase in the threat level is related to developments in Syria and Iraq where terrorist groups are planning attacks against the West,” May said, without offering direct evidence of those plans.

“Some of these plots are likely to involve foreign fighters who have traveled there from the U.K. and Europe to take part in those conflicts,” she said.

“The British public should be in no doubt that we will take the strongest possible action to protect our national security.”

She added: “We face a real and serious threat in the U.K. from international terrorism. I would urge the public to remain vigilant and to report any suspicious activity to the police.”

In what seemed a related development, Cameron told a news conference that new laws would facilitate the seizure of the passports of suspected British jihadis.

He said he would provide more detail in a statement to parliament Monday.

He drew a distinction between previous assessments that the main threat to British security came from al-Qaida, which has broken with the Islamic State because of its extreme tactics.

Security considerations at home are only a part of the challenge presented by the Islamic State advance. Britain has offered to arm Kurdish forces in northern Iraq and has flown surveillance missions in support of humanitarian airdrops. But President Obama has confirmed that he had asked Defense Secretary Chuck Hagel for options for military strikes in Syria, and there has been much speculation here that Britain may be asked for support, even though Parliament refused to authorize such action a year ago.

Cameron said British intelligence and security officials believed the number of Britons joining jihadist groups in Syria and, potentially, Iraq exceeded 500. By contrast, U.S. intelligence and law enforcement agencies have identified nearly a dozen Americans who have traveled to Syria to fight for the Islamic State.