A protester who swam into the Thames to disrupt this year's Boat Race could have been killed, four-time Olympic gold medal rower Sir Matthew Pinsent has told a court.

Sir Matthew, who was assistant umpire during April's race, made a written statement to police just hours after Trenton Oldfield, 36, was plucked from the path of the speeding Cambridge and Oxford boats.

Oldfield, of Myrdle Street, east London, denies causing a public nuisance but admits swimming in the path of the crews.

London's Isleworth Crown Court heard that Sir Matthew was immediately behind the university crews on a launch with umpire John Garrett. Behind them were another 25 motorised boats with officials, police, sponsors and camera crews.

Half way through the four-and-a-quarter mile race Sir Matthew, whose job was to look ahead of the crews, saw what he thought was a balloon in the water. He told Mr Garrett there was debris in the water ahead. But as they got closer Sir Matthew realised it was a swimmer who had chosen to swim into the path of the boats, he said.

In a statement read by prosecution barrister, Louis Mably, he said: "The risk for the swimmer was great, he could have been killed if he had been struck by an oar or the rigging which is metal. The incident caused me alarm as one of my primary roles is the safety of the competitors and public at large.

"I was worried about the safety of the swimmer. If he had been hit by an oar or boat he could have cracked his skull, his neck, fallen unconscious and drowned." Sir Matthew has previously competed in three of the university boat races and acted as an official in four.

Mr Garrett agreed the defendant could have been killed by the boats which were travelling at about 20mph. Mr Garrett, who halted the race waving a red flag and using a megaphone, said: "He put himself in very high danger and it wasn't just himself he was putting in danger. With so much traffic following on that tide there was a risk there may have been crashes between some of the boats."

Mr Garrett said there had been warnings of swimmers the previous year but nothing ahead of 2012's competition which Cambridge eventually won after a restart.