The Australian protestor who disrupted the prestigious Oxford-Cambridge university boat race on London's River Thames in April has been sentenced to six months in jail.

Anti-elitism campaigner Trenton Oldfield, 36, said he was trying to highlight inequality across Britain and the UK government's spending cuts.

A jury found him guilty last month of causing a public nuisance.

The judge said he spoiled the right of others to enjoy the race, and ordered him to serve at least three months behind bars.

Oldfield looked surprised as the sentence was passed down at Isleworth Crown Court in west London.

He had leaped into the river in a wetsuit and swam between the boats of Oxford and Cambridge universities in what he claimed was a protest against elitism.

It was the first time in the 158-year history of the annual rowing race between England's most prestigious universities that it has been interrupted by a swimmer.

Judge Anne Molyneux said Oldfield had acted dangerously and had failed to show what he was actually protesting about.

"You did nothing to address inequality by giving yourself the right to spoil the enjoyment of others," she said.

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"In doing so, you acted without regard for equality and contrary to the meaning of it.

"You made your decision to sabotage the race based on the membership or perceived membership of its participants of a group to which you took exception.

"You caused delay and disruption to it and to the members of the public who had gone to watch it and to enjoy the spectacle of top athletes competing.

"The rowers had trained for many months.

"You had no regard for the sacrifices they had made or for their rigorous training when you swam into their paths."

A statement from four-time Olympic rowing gold medallist Matthew Pinsent, who was in a launch near the incident, was read out in court.

"He could have been killed if he had been struck by an oar or the rigging, which is metal," the statement said.

His wife, Deepa Naik, spoke outside court.

"London today is the most unequal society in the Western World," she said.

"The criminalisation of protest and of protesters is a reflection of the hyper-anxiety of the bosses and the ruling classes."

Oldfield was supported by a group of well-wishers as he arrived at court and posed with a banner reading: "Stop criminalising protest".

ABC/AFP