Sources close to Speers expect the News Corp-owned channel to assert its contractual rights over their long-serving presenter.

Speers is poised to replace departing Insiders host Barrie Cassidy, who finishes on June 9. But a Sky spokeswoman confirmed Speers remains contracted to the 24-hour channel.

It is believed Speers has a two-year agreement with Sky, expiring at the end of this year, and he may be subject to a "non-compete" clause.

If enforced, this would prevent him from immediately joining a rival outlet.

"The announcement hit Sky like a bomb," says one source.

"Management aren't stupid; they know David has a 'halo' effect on Sky. He gets interviews with pollies from Labor and the Greens who'd never, in a million years, go on Peta Credlin or Andrew Bolt's [programs].

"Just look at Bill Shorten, who's made no secret of the fact he won't go Alan Jones' radio show. We're seeing more of that these days - politicians refusing to kowtow to certain media commentators."

On Friday, retiring Insiders host Barrie Cassidy all but confirmed Speers as his replacement.

"There has been no formal announcement and the bureaucracy gets in the way at times like this," Cassidy told ABC News Breakfast. "But he will be there - eventually."

Speers and Sky boss Paul Whittaker have been approached for comment. The ABC has not confirmed Speers' appointment, but senior staff say it's "a done deal".

"Until we hammer out the contractual issues, any announcement is a while away," says one employee.

While former Nine Network journalist Peter Stefanovic may take a permanent role with Sky, he is unlikely to replace Speers as political editor. (Nine is the owner of this masthead.)

Some Sky insiders believe Stefanovic will co-host a morning program with political reporter Laura Jayes.

An ABC manager says Speers was hired because "he's respected by all sides of politics ... as the station became more right-wing over the years, he [retained the respect of viewers and politicians]".

In a strategy dubbed "right at night", Sky has replaced the bulk of its evening news programming with conservative commentary.

Those close to the situation say Speers' defection is a "body blow" for the pay TV channel.

"He gave Sky News credibility," says media analyst Steve Allen.

"Even with all those right-wing loudmouths who appear at night, management could point to him [when defending accusation of bias]. The problem for Sky is not the hole that he leaves in their schedule. More than anything, it's a strategic loss."

Despite Sky's low ratings - most programs average fewer than 200,000 weekly viewers - Speers' interviews are frequently reported in other media outlets.

"The power of that amplification is a huge benefit for Sky," Allen says.

"You can't just stick someone else in his chair and hope for the same result. It takes years to cultivate the kind of contacts David has. When he leaves with his little black book, it'll be a loss for Sky."

Cassidy - a former press secretary to the late Labor prime minister Bob Hawke - founded Insiders in 2001.

While most established programs continue to shed viewers, Insiders has bucked the trend and grown its audience.

When Cassidy announced his resignation in March, speculation about his replacement centred on three ABC hosts: News Breakfast co-anchor Michael Rowland, and Radio National presenters Fran Kelly and Patricia Karvelas.

With Speers unlikely to join Insiders next month, they may be asked to fill the gap. On social media, all three congratulated their soon-to-be colleague.

"This is a superb appointment," Rowland tweeted. "A true class act. Welcome aboard, mate."

One ABC staffer says the decision has been welcomed inside the broadcaster's Ultimo headquarters.

"I've heard a couple of people say, 'You could never guess David's political leanings from his interviews'," he says.

"There's a real respect for his kind of political journalism."