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Using CS gas to disperse troublemakers during outbreaks of disorder is to be considered by the Metropolitan Police.

A Scotland Yard review into last summer's riots, entitled Four Days in August, says the Met is developing more "assertive tactics" to tackle disorder.

Some people felt police failed to help them or prevent properties being destroyed, the review acknowledges.

The force is already considering buying water cannon, while baton rounds are being made more readily available.

The review says the perceived police failings were largely down to not having enough officers on the ground. However, it acknowledges that more "flexible and dynamic" policing methods are needed.

It said the Met was reviewing tactics that are currently recognised by the Association of Chief Police Officers but "are not currently within its scope of options".

Among them, it describes the use of CS smoke as a recognised "last resort" tactic to disperse groups during serious disorder.

The gas is normally released from a grenade and is similar to tear gas.

CS smoke was used for the first time in Britain during riots in Toxteth, Liverpool in 1981.

But it is not part of public order policing situations and has "limitations" including the risk to innocent people, the report says.

It could also alter the mood of the crowd and damage community confidence, the review suggests.

Water cannon have been used in Northern Ireland but never in Britain and the review suggests it is "widely recognised as an effective tactic to disperse and distance aggressors".

It suggests they could have been successfully deployed on occasions including when trouble flared outside the Israeli Embassy in 2008 and 2009, and during the 2010 student protests.

However, at present, the Met relies on an agreement with the Police Service of Northern Ireland (PSNI) to have the resource available for use at 24 hours' notice.

The force is in discussions regarding the potential purchase of water cannon vehicles - at a cost of about £1m - to be based regionally in England and Wales and awaits the publication of Association of Chief Police Officers guidelines on the issue next year.

However, Met Commissioner Bernard Hogan-Howe has expressed doubts about their effectiveness and said their use must be a political decision.