Expert Rob Chadwick, of CGC Interiors and Outdoor Lighting, warned that certain flies lay their eggs down drains.

"The conditions inside sink pipes are ideal for flies to lay their eggs," Chadwick warned.

The British Pest Control Association (BPCA) said the small, dark-winged, non-biting gnats develop in standing water.

This is why they're a common sight in the home following a short or longer stint away on holiday.

They could also appear if you have slow-draining drains, as they can "live almost anywhere that water accumulates for a week or more".

These aquatic-loving species feed on bacteria and have a total lifecycle of around 140 days.

According to Chadwick, it's imperative to "cut the flies off at the source which, more often than not, is inside the sink pipes".

In order to remove the drain flies, Chadwick advised an "easy solution" by regularly pouring hot water down the drain every three to five days.

He said this would "effectively clean the drains and kill the eggs, larvae and any adult flies".

Chadwick added: "Others will recommend mixing baking soda and vinegar together, with half the soda to vinegar as a rough ratio, followed by hot water."

He is not totally for this option, however, as he's found that "hot water is usually more than sufficient".

The BCPA cautioned that "pouring hot water down the drain may [only] provide short-term control".

"Drain fly larvae are difficult to drown because they are able to trap air bubbles and remain submerged for a day or more," the BCPA added.

The BCPA noted: "The most effective method is to clean pipes and traps thoroughly to remove accumulated slime. Good housekeeping is key."

Spotting drain flies

Fantastic Pest Control pointed out that the flies are usually seen resting on the walls of bathrooms or hovering above sinks and drain holes.

Good housekeeping includes regularly cleaning the drains with boiling water, up to twice weekly, and common drain cleaners.

The chemicals found in commercial drain cleaners will kill the insects, as will bleach.