The number of people watching boxing and rugby league on television has dropped by a third during the past seven years, a survey carried out by Mori suggests.
The reason for the decline is said to be the growing number of matches being shown exclusively on digital channels, to which only two out of every five people have access.
The survey also reveals the number of people playing sport has dropped.
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Two out of every five people use or are members of a gym
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Mori
Head of Mori's sport research Peter Hasler said: "This is a worrying trend for sport and its sponsors."
The only sport to have increased its television audience is football.
Televised soccer is watched by 42% of people, a 4% rise.
And three out of every five of them are male.
The percentage of people playing sport has also dropped, the Mori poll of 2,167 adults suggests.
Play golf
The number of people playing rugby league has plummeted by 10%.
And snooker has experienced a decline of 9% that means only one out of every 20 people now pick up a cue - the same proportion that play golf.
Once again, though, football has avoided the decline.
One out of every 10 people still play.
Leisure pursuits
And swimming has seen a 33% rise, making it the most popular participation sport.
One out of every five people - and more than one out of every four of the women asked - now enjoy a dip.
This is part of a trend towards solitary leisure pursuits that require little organisation and can be done at any time, according to Mr Hasler.
"For example, two out of every five people use or are members of a gym."