Payne, a Beijing silver medallist, controlled the race throughout and finished in 2 hour 1min 37sec at Ostia.

The 21-year-old, who followed Tom Daly's diving triumph yesterday, held off an attack from Italian Martina Grimaldi, who drew level with the Briton on the final circuit, and surged off the final buoy to finish more than a body length ahead.

Russia's Ekaterina Seliverstova was the runner-up a second behind with Grimaldi taking the bronze.

Payne's victory followed the unexpected gold medal won by diver Tom Daley yesterday.

She said: "It feels amazing.

"I was anxious this morning.

"There was lots of pressure from the Games, winning a medal.

"And also I know my funding depends on it.

"But I know you don't just become a bad swimmer."

The South African-born swimmer was determined to go out in front and avoid the pack.

She said: "My plan was just to get out of the fight.

"You can waste so much energy.

"A few girls wanted to be there but I wasn't having any of it."

Payne admits the open water event – held in the sea today but also held in lakes and rivers – can be unpredictable with different conditions to contend with.

She has also put a lot of focus on sprint training and will take part in the individual medley over both 200 and 400 metres when the pool events begin next week.

She said: "Every one is different.

"But I've done a lot of training.

"I wasn't concentrating on the open water this year but this is my best chance of getting a medal."

Fellow Briton Katy Whitfield came 35th.