David Muir has been beaten in a vital ratings metric by NBC News’ new star Tom Llamas in an embarrassing blow to his bosses at ABC News. 

Muir, 51, was watched by 891,000 viewers in the 25-54 age group Tuesday night, while Llamas, 46, pulled in 904,000 viewers in the same demographic.

That age group is particularly prized by advertisers because of their relative youth and disposable income.

A win for Llamas, who just began at Nightly News on June 2, could result in advertisers moving their money from Muir’s World News Tonight to Llamas instead.

An industry insider told Daily Mail Tuesday that execs at ABC News were already unsettled by Llamas’s impact during his first month as host. 

Muir – whose World News Tonight remains the most popular nightly news broadcast in America overall – also found himself mocked for his choice of attire.

Journalist and podcaster Megyn Kelly laughed at the star for donning a ‘teeny tiny’ black t-shirt to report from devastating floods in Texas earlier this week. 

While black tees are a mainstay among male TV anchors reporting on location from warm locales, Muir’s was particularly tight and showed off his sculpted body to eye-popping effect.  

David Muir has lost to NBC Nightly News’ Tom Llamas in the 25-54 demo for the first time – an embarrassing blow in an emerging battle for younger viewers

An industry insider told Daily Mail Tuesday that execs at ABC News were already unsettled by Llamas’s impact during his first month as host

Kelly claimed Muir had purposely tightened his shirt to show off his ‘super, super tiny in the waist [and] super muscle-y in the arms.’ She pointed to what she framed as more professional attire from Llamas – before being bombarded with comments on social media that panned her for pettiness and hypocrisy.

Llamas’s black t-shirt was notably less clingy than the tee worn by Muir, who is known for posting smoldering photos of himself on his Instagram page. 

In the end, the former Fox firebrand branded Muir ‘the most vain man at ABC News’. 

Some of Kelly’s followers supported her criticism of Muir – but others suggested she was being petty and that the attacks on the ABC star were unwarranted. 

Muir’s clothing previously hit the headlines during the Los Angeles wildfires in January, when he was spotted wearing a yellow fire jacket that had been cinched in a the back with clothespins. 

Kelly and some other viewers went on to claim the fitted look was evidence of Muir’s ‘narcissism’, despite any proof.

Others said Muir was likely using the pins to clip a microphone or stop the clothing flapping and making a distracting noise while on air.  

In terms of ratings, Llamas is still a distant second to the more established World News Tonight host Muir. 

He took over from the well-liked Lester Holt and must now build his own loyal audience.

Llamas has seen a small ratings drop since taking over. But that dip is far smaller than the apocalyptic slump witnessed at CBS News, which viewers have deserted in droves since Norah O’Donnell quit as anchor in January. 

Muir’s show averaged 7.09million viewers Tuesday compared to NBC’s 5.81 million.

For the month, Muir reeled in 7.31 million viewers to NBC’s 5.67 million.

June was ABC’s biggest ratings win in three decades for the second quarter, continuing a trend of dominance started by Muir after he succeeded a then second-place Diane Sawyer.

But Llamas’ strides against Muir with winning over younger viewers will unsettle ABC News bosses who regard Muir as the network’s crown jewel. 

Muir recently emerged victor in a years-long feud with fellow ABC star George Stephanopoulos, who is best known for hosting Good Morning America. 

In January, Muir was accused of being a vain narcissist after he was caught wearing a fire jacket cinched in with clothes pins while broadcasting from the LA fires. But others wondered whether he was trying to keep a microphone secured or stop the garment flapping 

Megyn Kelly has claimed David Muir is ‘at it again’ when it comes to the World News Tonight anchor formfitting fashion choice

New figures also show that Llamas is gaining viewers among 18 to 49 year olds.  

The gains for Llamas are more impressive given they’ve come during a quieter news cycle after last year’s presidential election and Olympics, which help drive bumper ratings to news shows. 

A media source Friday told Daily Mail that NBC News are quietly confident Llamas has got what it takes to topple Muir from his no.1 ratings perch. 

‘There’s a quiet optimism that Tom has what it takes to eventually edge out Muir in the demo but no one’s taking a victory lap just yet,’ the source said.

‘The team is taking one day at a time and is focused on putting on the best newscast each night.’

Llamas, 46, told The Washington Post in June something similar, while explaining how he ‘want[s] to be number one’ in primetime.

‘It’s not easy,’ Lamas said. ‘But it’s something I think we can do.’

He admitted, ‘It’s not going to happen in a month.’

Another insider sought to pour cold water on Llamas success – and Kelly’s targeting of Muir.

That source said: ‘Honestly, the reality is Muir just had the biggest win over NBC in 30 years in June with Llamas as anchor. 

‘It took a month and a half for NBC to get a night in the demo, something Lester Holt occasionally used to do, too – but Holt didn’t try to get headlines for it. It’s silly.”

‘With Muir in Texas, “World News Tonight” beat NBC’s Llamas by nearly 2 million viewers Monday. The nights with major news are really the most telling.’

‘It’s clear the way Megyn Kelly gets attention is by trashing others,’ the source added.

‘She does this for clicks and headlines from behind her studio microphone, while others are actually out there reporting on people who’ve endured devastating loss.’