British Security officials believe they have thwarted an alleged plot to assassinate Theresa May by terrorists who would first bomb their way into Downing Street and then kill the prime minister, it has emerged.

Two men were arrested last week following a joint operation by MI5, the UK's counter-terrorism security service. One of the arrestees, Naa'imur Zakariyah Rahman, identified himself as "Bangladeshi British” and the other -- Mohammad Aqib Imran -- is a "Pakistani British".

Security officials believe the alleged Islamist plot is the ninth to have been thwarted since March this year.

The alleged details of the latest plot emerged as an official report into the terrorist attacks Britain suffered between March and June was released, showing that MI5 had intelligence that could have stopped the Manchester terror attack in May but, with the benefit of hindsight, did not correctly interpret it.

Two men, aged 20 and 21, were detained on 28 November following an operation involving MI5 investigators and counter-terrorism police officers from Scotland Yard and the West Midlands.

They made their first appearance yesterday at Westminster magistrates court in central London. They were charged with plotting to assassinate May in a bomb and knife attack, reported AFP.

Naa'imur Zakariyah Rahman is accused of planning an explosion outside May's Downing Street office then trying to enter the building with a suicide vest and a knife in a bid to kill her.

State prosecutor Mark Carroll said Rahman planned to detonate an improvised explosive device at the Downing Street gates, then, in the ensuing chaos, try to enter the building "with a view to trying to kill the prime minister".

He planned to use a suicide vest, pepper spray and a knife, Carroll told Westminster Magistrates Court in London.

Rahman had carried out hostile reconnaissance of the area as part of his preparation, it is alleged.

Rahman is charged with preparing terrorist acts and also with assisting another man, Mohammad Aqib Imran, to prepare separate terror acts.

He appeared in court alongside Imran, 21, who is accused of trying to join the Islamic State jihadist terror group by trying to obtain a fake passport with a view to travelling to Libya.

Rahman spoke only to confirm his name and date of birth. He said he was of no fixed abode and said his nationality was "Bangladeshi British. Imran said he was "Pakistani British".

Chief Magistrate Emma Arbuthnot remanded Rahman and Imran in custody to appear for a short hearing at England's Old Bailey central criminal court on December 20.

The court appearance came a day after Home Secretary Amber Rudd told parliament that 22 Islamist terror plots had been thwarted since the killing of a British soldier on a London street by two Islamist extremists in 2013.

No. 10 Downing Street is the official residence of British prime ministers. It is heavily guarded and there is a gate at the end of the street preventing members of the public from getting close to the house, reported Reuters.