A senior Whitehall official said the guidance was being drawn up as part of a drive to use persuasion rather than the criminal justice system to fight extremism.

He added: “The aim is to stop people being dragged into extremism.

“We are not talking about letting someone off who has committed a clear offence, but where it is unclear if an offence has been committed.

“For instance, where there has been incitement or someone has been on the internet there can be a grey area where there is some discretion and it would be more sensible to avoid going down the criminal route.”

The Government’s counter- terrorism board is drawing up the advice, which will be sent to all police forces, including the Metropolitan, later this year.

The move follows an updated Home Office counter-terrorism strategy announced earlier this year. The new strategy urges preventative measures to win round potential extremists instead of arrest and prosecution.

“We need to be able to provide support for individuals who are drawn into criminal activity,” the document says.

Councils, community groups and the Government’s youth justice board will be among organisations expected to identify those drawn into extremism or at risk.

Social workers, teachers and other professionals will be asked to try to work with some Muslim youths to reduce the likelihood of them turning into extremists.

But the new strategy is likely to reduce the likelihood of prosecutions against Islamist extremists protesting against troops.

In Luton earlier this year, protesters displayed placards bearing the words “butchers” and “animals” at a homecoming parade for 2nd Battalion, The Royal Anglian Regiment. There were no arrests for incitement.

A Home Office spokesman said: “Preventing people becoming radicalised is a key priority for the Government. The police response needs to be proportionate to deal with crimes people commit while reducing the risk to public safety.”

The latest move represents a reversal of the policy introduced under Tony Blair in the wake of the terrorist attacks in London in 2005, when as Prime Minister he called for an overhaul of the criminal justice system to root out and prosecute extremists.

Past attempts to win over potential Muslim radicals have frequently run into controversy. Millions of pounds have been pledged to fund Muslim groups, drawing claims that they are receiving special treatment.