A giant poster urging Sainsbury's staff to encourage customers to 'spend an additional 50p during each shopping trip' has mistakenly been displayed in the window of a London store.

The '50p Challenge' was spotted by passer-by Chris Dodd, a TV freelancer, who proceeeded to whip up a social media storm by posting a picture on Twitter with the caption: "Not sure this is supposed to be in your window @sainsburys..."

A rather amusing conversation then follows on Twitter with Sainsbury's representatives desperately trying to find out which store the poster is being displayed at so they can order its instant removal - 'because it was clearly meant for staff' - only for the Twitter user to refuse to reveal its location.

The store was later identified as the Romford Road branch of Sainsbury's in Stratford, east London.

Twitter users were quick to ridicule the poster, with a former Waitrose employee writing that the rival would "die of mortification" if a similar slip-up occurred.

Another wrote: "No wonder your customers are flocking to Aldi in their droves @Sainsburys?"

Some had suggestions for how Sainsbury's could secure the extra spending, such as "Double swipe a 50p item at the checkout maybe?", and "When customers are not looking, just stuff a few extra items into their basket".

A Sainsbury's spokeswoman said: "We often use posters to make store targets fun and achievable for our colleagues.

"They are intended for colleague areas in the store, but this one was mistakenly put on public display."

So now we know what really goes on in that staff canteen...

Here's the original tweet, which has been re-tweeted more than 3,000 times: