The former prime minister - who once blamed "newt counting" for holding up "the productivity and the prosperity of this country" - applied to install the 11m-by-4m outdoor feature at his Oxfordshire manor house, where he lives with his third wife and their three children, back in June.

But the process may be delayed after the local countryside officer warned of the risk to great crested newts - which thrive in the village and are protected under the Wildlife and Countryside Act 1981.

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In a report submitted to South Oxfordshire District Council, Edward Church said the location of the planned pool "falls within the red zone of highest risk" to the species.

"Natural England guidance requires that proposals need to demonstrate no risk to [great crested newts], or appropriate levels of mitigation and compensation following assessment," he added.

"Based on the information available to me currently, I am of the opinion that there is a reasonable likelihood that [great crested newts] are present and could be impacted by the proposed development."

Mr Church said protected species surveys were likely to be needed for the application to go ahead, and he offered to visit the site to "ground-truth this desktop assessment and view the suitability of impacted habitats to provide the applicant with assurances that this is the correct approach".

But, in the meantime, the countryside officer could not recommend planning permission was granted.

The Wildlife Trust says the great crested newt is the biggest of the UK's newt species, measuring up to 17cm.

The so-called "warty newt" is almost black with spotted flanks and an orange belly, with the charity comparing it to a mini-dinosaur.

Sky News contacted Boris Johnson's office, but his spokesman refused to comment.