Walkers who found one horse drowned and three others scrambling to escape a flooded Bushey field used dog leads to keep the animals afloat, it has emerged.

One of the horses had already perished by the time the passers-by discovered the distressed animals in the field off Merry Hill Lane at about 8.40am.

Emergency services were called in to rescue the stricken beasts, which had fallen into a large pool of water created by a burst underground pipe in the Woodland Trust-owned land.

Police and the RSPCA are investigating why the animals were in the field, which is not used to keep horses, at the time the pipe burst.

A veterinary nurse called in to treat the surviving horses described the events as “a tragic accident”.

Dogwalker Jane Krite, who passed the scene later that morning, said: “These horses shouldn’t be kept here. Apparently they were treading water for most of the night – it is very distressing. Every year there are horses put on the land, it is a crime – they are not looked after.

“It is awful when you go down there and hear the horses whinnying. It is dangerous to keep them here and it shouldn’t happen. It is just terrible, those poor animals.”

Firefighters managed to pull the three surviving horses, which were up to their necks in the water, out of the muddy pool by fixing a harness around them and guiding them to solid ground.

Watch commander Dave Elliott, from Watford station, said one of the horses was close to slipping under and a walker had to hold its head up while firefighters made their way to the field.

He said: “They were in a very bad state. If the member of the public had not been there they would have drowned.

“They were very, very distressed and very weak. They had been in there, we believe, over night.”

Workmen from Veolia Water, which owns the pipe, were on site this morning attempting to stop the flooding.

A company representative said: "We do not know the exact size of the main but it was one of our larger trunk mains - one used to transport water and not used for direct water supply - and it caused a small surface area, approximately three metres wide, to dip into the ground around the burst, creating a hole.

"We are very much saddened by the consequence of this morning’s events and currently have our representatives on site to tend to the burst. Customers will not be affected by a loss of water supply."