Keeley Hawes will go head-to-head with the legacy of William Makepeace Thackeray this weekend, when the BBC puts its flagship drama Bodyguard up against ITV’s new adaptation of Vanity Fair in the peak 9pm Sunday night viewing slot.

But unlike previous scheduling battles they could both end up as winners, with millions of viewers expected to watch both programmes thanks to catch-up services.

The BBC took advantage of the bank holiday weekend to screen the first two episodes of Bodyguard in quick succession on Sunday and Monday nights, in an attempt to lock in the audience for the drama about a home secretary and her protection officer, written by Jed Mercurio.

The tactic – also used to launch the BBC adaption of JK Rowling’s Strike novels last year – worked and Bodyguard attracted 6.7 million viewers for its first episode, the highest overnight audience for a drama launch this year, and retained most of that number the following night.

Although catch-up figures are not released for several weeks after broadcast, similar BBC dramas have attracted millions more viewers on iPlayer, meaning the final consolidated audience could be substantially higher.

ITV will attempt to fight back by showing the first two episodes of Vanity Fair on Sunday and Monday evenings. The show, starring Olivia Cooke and Tom Bateman, with other roles played by Martin Clunes and Frances de la Tour with special appearances from Michael Palin, will then compete with Bodyguard in the Sunday night slot for the rest of its run.

Recent research by the media regulator, Ofcom, found a substantial decline in the number of big-audience shows – large sporting events such as the World Cup aside – which can regularly attract more than 5 million viewers to live broadcasts, with long-running dramas such as Holby City among the worst hit.

The first two episodes of Vanity Fair will air on Sunday and Monday evenings. Photograph: ITV

It is forcing stations to increasingly rely on big-budget dramas such as Bodyguard and Vanity Fair – following on from previous such hits such as Call the Midwife or Broadchurch – to reach the biggest audiences.

“Premium drama remains the crown jewel content for broadcasters and streaming services. It has become a fiercely competitive arena over recent years, and its importance has only increased as the deep-pocketed Netflixes and Amazons of the world have entered the fray,” said Alex Fenton of the media consultancy Enders Analysis. “The competition for eyeballs has opened up and driven up production costs and standards, and bigger, better dramas are a good way for content providers to differentiate themselves.”

He said there was still a substantial value for traditional channels to make appointment television that brings in viewers at a set time every week. “ITV tends to skew older in its viewership, but younger viewers tend to be the most valuable demographics to advertisers, so any content which can reassure younger people of the relevance of broadcast television programming is a big win for them.”

The cost of producing expensive big-budget shows can often be shared with streaming services. Vanity Fair has been made by an ITV-owned company in partnership with Amazon, which has the rights to show it on its Prime Video service outside the UK. This enables ITV to reduce the size of its gamble and bank the profits through its advertising arm.

But senior executives at last week’s Edinburgh television festival privately expressed concern that the likes of Amazon and Netflix could increasingly choose to go it alone and handle all the costs themselves.

Although commercial broadcasters can make a similar amount per view from catchup TV as from selling broadcast ads, not all is rosy, with some UK channels reporting that even as audiences are increasingly shifting online, the overall pool of advertising cash is not moving with them at the same speed.

Either way, in this latest Sunday night battle there is going to be one clear winner: Bodyguard is made for the BBC by World Productions, a company owned by ITV. No matter who tops the ratings, the commercial broadcaster will benefit.