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It appeared to come from the Lewisham police account - a subsidiary of the Metropolitan Police - although it didn't have a Twitter verified "tick".
The following reasons were cited:
There are only three leaves in the shamrock - Ireland's national symbol
Some people said it implied inherent criminality in Irish people
It appears to have been nabbed from an anti-drink driving campaign in the US (which doesn't look great on behalf of a police force)
That may also also explain why the picture is so pixelated
Others wondered if the police could be using its time more effectively
A spokesperson for the Metropolitan Police told indy100 its social media team were looking into the tweet and would post an update on the account if they deemed it necessary.
Moments later it was deleted and replaced by this...
More: Can you spot the obvious problem with this St Patrick's Day poster?