Jihadis Planned to Bomb Britain's Downing Street and Assassinate Prime Minister Theresa May

British Prime Minister Theresa May leaves 10 Downing Street, in London, to greet Spanish Prime Minister Mariano Rajoy, on December 5. British security services have foiled an alleged plot to kill the British prime minister, according to media reports. Jack Taylor/Getty

British security services have foiled an alleged plot to kill British Prime Minister Theresa May, according to media reports.

London's Metropolitan Police released a statement Tuesday announcing the arrest of two men last week on terrorism offenses.

One of those, 20-year-old Naa'imur Zakariyah Rahman, is accused of planning to bomb the gates of May's Downing Street residence, in central London, before attacking her with a knife in the ensuing carnage, The Times reported. British counterterrorism police arrested Rahman in a November 28 raid.

The other suspect, 21-year-old Mohammed Aqib Imran, is accused of plotting and preparing terrorist acts overseas. Both are to appear in court Thursday.

It remains unclear for which extremist group, if any, the pair were acting.

Britain has suffered four attacks claimed by the Islamic State militant group (ISIS) this year: a car-ramming attack on Westminster bridge in March, a suicide bomb blast at an Ariana Grande Manchester concert in May, a van-ramming and knife attack on London Bridge in June and a partial explosion on a subway train in West London in September.

The attacks killed 35 people and wounded dozens more. In another extremist attack, a man ran down Muslims outside a mosque in North London in June, killing one.

Former Prime Minister Margaret Thatcher introduced fortified gates at the Downing Street residence in 1989 as a security precaution. The Irish Republican Army, or IRA, launched a mortar at Downing Street in 1991, but it caused no damage. The group previously targeted Thatcher at her Conservative Party's conference in 1984. The bomb blast missed her but killed five people, including one of her fellow British politicians.

London Bridge attack
A man is seen on the ground after armed police officers opened fire on suspected attackers in Borough Market, near London Bridge, on June 3. Reuters/Gabriele Sciotto

Britain is dealing with one of the worst cases of homegrown radicalization in Europe, alongside France. The chief of Britain's MI5 domestic security service warned in October that the threat of jihadi attacks in Britain was at its "highest tempo" in his three-decade career.

British security services have foiled nine attack plots this year already, including the one targeting May. In the four years since British soldier Lee Rigby was hacked to death in southeast London, MI5 and police have foiled as many as 22 plots by Islamists and right-wing extremists.

Manchester attack
A woman looks at the flowers for the victims of the Manchester Arena attack, in central Manchester, England, on May 23. Stefan Wermuth/Reuters

Of those plots that have been successful, one was a vehicle-ramming outside a mosque in North London that left one person dead.

The threat level in Britain remains at "severe," meaning that an attack is likely. Some 800 British nationals traveled to fight for ISIS in Iraq and Syria, and officials estimate that more than half of those have returned to the country.

MI5 is dealing with as many as 500 counterterrorism investigations, more than 3,000 individuals of interest and a wider pool of 20,000 people who have been probed for potential extremist links or ideological beliefs.

In a report released Tuesday, a review of the British intelligence services and their handling of information and cases before the Manchester attack in May said that it was "conceivable" that the suicide bombing could have been stopped.