A 20-year-old man accused in a plot to assassinate Prime Minister Theresa May was remanded in custody Wednesday during an appearance in London's Westminster Magistrates' Court.

Naa'imur Zakariyah Rahman has been charged with preparing acts of terrorism and appeared alongside another man, Mohammed Aqib Imran, who is accused of trying to join ISIS, but wasn't charged in connection with the assassination plot. Imran was also in court Wednesday.

The plan allegedly involved planting a bomb near the entrance of Downing Street and then continuing the attack with a knife and suicide vest in a bid to kill the U.K. leader in the ensuing chaos.

Rahman is also accused of assisting Imran in terror planning.

The two were arrested in London and Birmingham on Nov. 28 by the Metropolitan Police Counter Terrorism Command.

Britain's media had reported earlier that two men were involved in the plot to kill May.

A Downing Street spokesperson declined immediate comment on the report.

Britain Terror Plot

Naa'imur Zakariyah Rahman, left, and Mohammed Aqib Imran appeared in Westminster Magistrates' Court in London on Wednesday. (Elizabeth Cook/PA via AP)

Andrew Parker, head of the country's intelligence services at spy agency MI5, said recently that the threat from militant attacks was the highest he had seen in 34 years of espionage.

Four attacks this year left 36 people dead in Britain, making it the deadliest year since the London transit bombings of July 2005.

Manchester suspect missed opportunities

An independent review of counterterrorism performance ordered by Home Secretary Amber Rudd revealed on Tuesday that one of those attacks — the May 22 concert bombing in Manchester that left 22 people dead — might have been thwarted "had the cards fallen differently."

The report from lawyer David Anderson, who has consulted on modern terrorism laws in Britain, found that three extremists involved in the four attacks in Britain this year had at some point been investigated by counterterrorism police or security services.

Nonetheless, he credited police and the MI5 domestic intelligence service with stopping most attacks at a time when Britain faces an unprecedented level of extremist activity.

"MI5 and counterterrorism policing got a great deal right – particularly in the case of Manchester, they could have succeeded had the cards fallen differently," Anderson said.

The reports said Manchester bomber Salman Abedi wasn't being actively investigated when he detonated a suicide device, although he had been scrutinized in the past as a "subject of interest."

But Anderson said MI5 obtained intelligence in the months before the attack that might have led to an active investigation of Abedi "had its true significance been properly understood."

The report says two pieces of information came to MI5 indicating possible criminal activity by Abedi, but this wasn't judged to be related to possible terrorist acts.

"In retrospect, the intelligence can be seen to have been highly relevant to the planned attack," Anderson said.

He also found that counterterrorism officials didn't place a so-called ports action on Abedi after he travelled to Libya in April. That was a missed opportunity, the report says.

"This would have triggered an alert when he returned shortly before the attack, which could have enabled him to be questioned and searched at the airport," Anderson said.

Anderson said it wasn't clear whether such an investigation would have led to Abedi's plan being prevented, and that MI5 believes it wouldn't have thwarted the bomber.

With files from Reuters