Dr Jeroen Ensink, 41, was stabbed to death by Femi Nandap last December as he left his flat in Islington, north London, to post cards announcing the birth of his daughter Fleur.

When the academic failed to return home, his wife, Nadja Ensink-Teich, went outside to find police had cordoned off the street and the cards her husband had been carrying strewn over the pavement.

Nandap, resident of Woolwich, south London and who had been studying at the London School of Economics,admitted manslaughter by diminished responsibility and was handed a hospital order without a time limit.

It later emerged that Nandap, a Nigerian student whose visa had expired, had knife and assault charges against him dropped six days earlier.

The Old Bailey heard how Nandap developed mental health problems after he began smoking a large quantity of cannabis while studying in Boston in the United States in 2012.

Two years later he moved to London after gaining a place at the School of Oriental and African Studies (SOAS).

But last May he was arrested in Edmonton, north London after punching and biting a police officer. When he was arrested he was found with two kitchen knives.

He was charged in connection with the alleged offences but was granted permission to return to his native Nigeria in June on condition he returned to the UK in August to attend court.

When he failed to appear his sister, who also lives in London, provided police with a letter detailing treatment he had been having for his mental health conditions.

After returning to the UK he appeared in court but despite having breached his original bail conditions, was not remanded into custody.

He was was never prosecuted for the bail offence, and the CPS dropped the knife and assault charges on 23 December due to ‘insufficient evidence’.

The prosecution admitted that the decision to drop the charges six days before the fatal stabbing had been wrong.

Judge Nicholas Hilliard QC said Dr Ensink, who had devoted his career to helping improve water sanitation for the world’s poor, was a truly “remarkable man”.

The judge added: “I express the hope that those in a position to do so will investigate all aspects of this case and the appropriate lessons will be learned.”