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Islamic commentators have claimed that the massively popular # YouAintNoMuslimBruv hashtag is forbidden by sharia law.

This phrase went viral on social media over the weekend in the wake of a suspected terror attack at Leytonstone tube station on Saturday.

A video showed a man pinned to the floor after being tasered by police , with a bystander shouting: "You ain't no Muslim bruv."

Now dozens of Islamic people have taken to social media to warn that declaring someone a non-Muslim is forbidden by ancient Islamic legal guidelines.

The act of describing someone as an apostate (someone who has left the religion) is called " takfir " in Arabic.

Some scholars believe this is so grave a sin that it is punishable by death, although different schools of Islam differ on this point.

The concept of Takfir dates back to a hadith (sayings of the Prophet Mohammed) in which he said: "He who says to his brother 'O disbeliever', then it returns upon one of them."

It is important to understand that a Muslim who criticises anyone using the hashtag does not necessarily support the crime, but is concerned about the dangers of excommunicating anyone from the faith.

(Image: Kibzzy Free-gaza Rahman)

The so-called Islamic State have drawn criticism across the Muslim world because they repeatedly justify their atrocities against members of the same religion by describing them as apostates.

Bilal Ahmed, a writer and activist, wrote a blog post explaining the religious concerns around the practice of takfir .

"For all the relief that we may feel at someone taking care not to automatically associate Islam with terrorism, I am deeply alarmed at the fact that we feel this comfortable engaging in takfir ," he wrote.

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"Takfir is a term that essentially describes situations where a Muslim’s legitimacy is being questioned.

"It refers to a practice of exclusion, whereby a Muslim is declared to be somehow impure, and an illegitimate member of the religious community.

"These days, it is used to refer to salafi [fundamentalist] and militaristic strains of Islamic practice, which rely on moralist fervour about legitimate and illegitimate Muslims."

Muhaydin Mire, 29, of Sansom Road, East London, arrived at Westminster Magistrates' Court today charged with the attempted murder of a 56-year-old man.