TNT Sports lost five million viewers compared to last year's Champions League final following their decision to put the match behind a paywall.
The broadcaster held off on calls from Prime Minister Sir Keir Starmer to make the game free to watch, as it has done in the past.
Some 12.6 million people tuned in for Liverpool's final against Real Madrid in 2022, when predecessor BT Sport broadcast the game free of charge.
The figure is the highest ever achieved for BT Sports, with only seven million turned in for the final this year.
TNT did not say how many subscribed to them on the back of them standing firm with their paywall.
The broadcaster was offering a one-off payment of £4.99 to HBO for a monthly subscription to watch the match.
A TNT spokesman said: "This year's three European club finals were watched by over 13 million viewers across TNT Sports and HBO Max.
"The UEFA Champions League final performed strongly in the UK, reaching more than seven million people and delivering TNT Sports' 2nd highest ever share of total TV audience... consistent with Manchester City vs Inter Milan from three seasons ago.
TNT Sports lost five million viewers compared to its record audience
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"Across the season, Uefa club competitions attracted larger audiences on TNT Sports with average audiences up five per cent compared to last season."
Millions chose to find a different way to watch the final, with research from the Guardian reporting there were 16.2 million views on illegal streams in the UK after it was not made free-to-air.
Sir Keir and London Mayor Sir Sadiq Khan called for the paywall to be dropped.
Sir Sadiq wrote last week: "Football is at its best when it brings people together. Supporters should not be priced out of watching one of the biggest matches in the world. I support the Prime Minister's call for the Champions League final to be made free to watch."
Seven million people tuned in to see PSG win the Champions League
| REUTERSThe Prime Minister, a keen Arsenal fan, also previously wrote: "I was saddened to see that, for the first time since the competition started 34 years ago, TNT Sports has decided that [the fixture] will not be free to watch for football supporters here in the UK.
"I am a firm believer that the final of this competition should remain free to watch, whether Arsenal have made it or not.
"Obviously, I want as many fellow fans as possible to be able to watch our team in this historic final for the first time in 20 years. However, this is bigger than that.
"This is about supporters of all teams coming together in living rooms and pubs in every corner of the country to watch the most elite players in Europe battle it out.
TNT ignored the Prime Minister's calls
| GETTY"Hard-working people should not have to worry about forking out for a subscription to watch a game of this magnitude.
"We should be putting supporters first. That is why I have already encouraged Fifa to do more to make tickets more affordable at this summer's World Cup.
"That is also why I want to strongly urge you to reconsider and make the final next Saturday free to watch for the millions of passionate football fans in this country."
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