San Francisco Airport has warned taxi drivers to stop urinating and defecating outside their cars.

Furious officials sent a letter to drivers last week saying they have had to increase security patrols to prevent the streets outside the airport becoming a 'health hazard'.

It is believed taxi drivers are reluctant to go to the bathroom for fear of losing their place in the highly lucrative airport taxi queue.

One former taxi company owner said he was shocked to receive the email and couldn't believe this was happening. Pictured: San Francisco Airport

Furious officials sent a letter (pictured) to drivers last week saying they have had to increase security patrols to prevent the streets outside the airport becoming a 'health hazard'

The letter from Seth Morgan, a senior transportation planner at SFO, said: 'Airport staff have noticed an increased rate of urination and defecation in the rear of SFO Taxi Lot #3 and nearby stairwells.'

'This creates a highly unsanitary condition and a health hazard for SFO custodians and other staff handling the materials stored in these areas.'

It added: 'Regular patrols through this area are being added both during the day and overnight, including during open lot hours.'

The letter said restrooms are open 24/7 and drivers can leave their cars in designated parking lots while they go.

One former taxi company owner said he was shocked to receive the email and couldn't believe this was happening.

John Lazar, the former owner of Luxor Cab Co., told the San Francisco Examiner: 'I don't think our cab drivers are so degenerate they piss and sh*t on the streets.'

He said it would be easy for the airport to enforce the rules by sending drivers to the back of the line if they are caught out.