A Czech-Polish team said Friday it had discovered the world's deepest underwater cave in the eastern Czech Republic.

At 404 metres (1,325 feet) deep, the Hranicka Propast, a limestone abyss near the city of Hranice, beats out Italy's Pozzo del Merro cave that is 392 metres deep for the world record, Miroslav Lukas of the Czech Speleological Society told AFP.

"We wanted to beat the Italian record. We succeeded and now we have the magic number of 404 metres," said Lukas, insisting the cave was "definitely" deeper than that.

"I don't know if it's by five metres or a hundred, but the depth is set to change," he said.

Lukas said the figure was confirmed by a depth gauge carried by a robot and by the length of a data cable connected to the robot.

On September 27, Polish diver Krzysztof Starnawski first descended to 200 metres to install a signal line for the robot, then navigated the device down to the record depth.

"The robot broadcast information about its depth and course into a monitoring device on the surface," Lukas explained to AFP.

"It fell deeper and deeper, heading into places where we could see no bottom, until it reached 404 metres," he added.

Lukas said the project was co-financed by the National Geographic society, which broke the story online late Thursday.

Research at Hranicka Propast started in 1963, said Lukas, who first dived there in 1974.

In 1995, a robot reached a depth of 205 metres in the abyss, setting the deepest point.

In 2012, Starnawski "discovered a narrow opening at 200 metres and a large break" that allowed him to reach a depth of 373 metres. Two years ago he reached 384 metres.