The landlord had given his tenants four weeks to vacate the flat after evicting them, and checked on the property after this time, only to discover the horrific sight and awful smell.
A surveyor from London Network for Pest Solutions was immediately called in to carry out an inspection of the flat.
In a blog post about the incident, the pest control company wrote: “The smell was unbelievable, as the pigeons had done a brilliant job at redecorating, covering everything in guano.
“The landlord took one peek inside the property, smelt an overpowering stench, left and immediately called us.
“Our surveyor immediately visited the site in full PPE. This included a powered respirator due to the health and safety risks presented by the birds and their guano.
“Quite simply, the patio door had been left open after the eviction. In just four weeks, the flock of pigeons had discovered the entrance, realised it was a good place to roost and set about doing their best impression of the artist Jackson Pollack.”
The pest control company said it was the worst case staff had ever seen of this type.
London Network for Pest Solutions said the entire flat will need to be emptied, cleaned, decorated and refurnished, with the landlord estimating the cost will be £15,000.
The figure includes the cost of replacing furniture, cleaning and the amount of revenue lost while the flat cannot be occupied.
Pigeons are London’s most common bird pest, followed by the starling, house sparrow and seagull.
One pest control company, Integrum Services, estimates there are around 2,993,000 pigeons in the capital and one to every three people in the UK.
They are known to carry several potentially infectious diseases, including ornithosis, tuberculosis and salmonella.
DIY bird removal and proofing is therefore not recommended, the London Network for Pest Solutions said.
The installation of bird netting, a spikes system and nest removal are all options used by pest control professionals.
Hawking is also an option, which involves using trained birds of prey to deter pests without harm.