In a statement, Testwood Lakes said it was “extremely concerned” to hear of the “serious incident”.
“Hampshire & Isle of Wight Police has confirmed that a multi-agency operation is under way following a report that a 15-year-old boy is missing after he was last seen swimming at Testwood Lakes,” it said.
“We have taken the decision to close the nature reserve until further notice and join the police in asking members of the public to please avoid the area at this time.”
reporter Hannah Walsh, speaking from the scene, said: “The road into Testwood Lakes is closed, with a police closure sign and cones in place.
“A number of cars have pulled up to collect people – possibly at the lakes before it was evacuated – and many of them have been sat opposite the lake entrance on a grass area in the shade, trying to stay cool.”
She added that although the first wave of emergency service vehicles had left, two search and rescue units had now arrived.