The idea, which has been dubbed 'SatLav', is the brainchild of Gail Knight, a 26-year-old student from Clapham Junction who won first prize in Westminster council’s innovation competition.

The mobile phone text service lets users send the word "toilet" to 80097 to pinpoint their location and find the nearest public loo.

Users pay the 25p for each text, with Westminster council picking up the extra 9p on top of that.

Toilets in the scheme include the council's 40 public toilets, the GLA's facilities, London Underground stations and major department stores in the capital.

The plan is to cut down on the amount of "street urination".

"Up to 45,460 litres of urine is at risk of ending up in the city's streets and alleyways through irresponsible and anti-social behaviour," a statement said, basing its piss-count on the amount of liquid collected from the borough's urinals.

"From today onwards nobody should ever get caught short again, and we understand how important that is, be it for a young mum with children in tow, older people or friends on a shopping trip or night out," said Westminster councillor Alan Bradley.