Netflix is to release its long-awaited documentary series on the disappearance of Madeleine McCann on Friday, despite opposition from the missing child’s family and TV industry speculation that programme-makers failed to gain access to key individuals involved in the case.

The US streaming service first commissioned the programme in 2017, as interest grew hugely in true crime and cold case TV programmes following the success of Making a Murderer.

However, despite spending enormous sums to produce eight hour-long episodes, its release has been repeatedly delayed, raising speculation over what, if anything, the show has uncovered, and the state of behind-the-scenes wrangling over its content.

Madeleine McCann went missing when she was three during a family holiday. Photograph: PA

Kate and Gerry McCann, whose daughter went missing in 2007 when she was three, while on holiday in Praia da Luz, Portugal, have repeatedly refused to take part in the show. They have also urged those around them to resist efforts by London-based Pulse Films, which is making the programme on behalf of Netflix, to get them to give interviews.

Clarence Mitchell, the family’s former spokesman, who still assists with media inquiries, told the Guardian: “Kate and Gerry and their wider family and friends were approached some months ago to participate in the documentary. Kate and Gerry didn’t ask for it and don’t see how it will help the search for Maddie on a practical level, so they chose not to engage.”

Instead, the programme is expected to lean heavily on interviews with the Portuguese officials who originally investigated the case, many of whom have since established media careers discussing the incident.

Production staff are thought to have interviewed more than 40 individuals, although some leading Fleet Street journalists who covered the story at the time have said they declined to take part.

Those who are thought to have given interviews include the Portuguese detective Gonçalo Amaral and the journalists Anthony Summers and Robbyn Swan, who wrote a book on the case. Others include people questioned by the Portuguese police such as the Briton Robert Murat and the Russian Sergey Malinka, plus the child protection experts Jim Gamble and Ernie Allan.

Some associates of the McCanns did take part. Brian Kennedy, the millionaire businessman who helped fund the initial search for Madeleine, has also talked to the show.

The McCanns have kept a lower profile in recent years, being selective in their media appearances. Despite £11.75m being spent by the British police, there remains no sign of Madeleine after 12 years, although the Metropolitan police have continued to fund Operation Grange until the end of this month.

Earlier this month, the programme was listed on Netflix’s new release schedule for 15 March, only for the show to be removed from the public list. Although the Netflix programme is subject to delay, it is expected a trailer will be released on Thursday, with the rest of the show to follow a day later, concluding a sometimes torturous route to screen for a programme that was at some points speculated to have vanished for ever.

In line with Netflix’s secretive style, the show was never formally announced as a commission, aside from speculation in the industry press. The streaming service also has a tendency to drop shows with a minimum of advance publicity, instead relying on the power of its home screen’s algorithm and word-of-mouth publicity to spread awareness of its new shows.

However, individuals with knowledge of the production say a trailer was due to be released last week, only for that to be pulled at the last minute.

British documentary makers are also looking enviously at the programme’s production values and wondering what Netflix will have to show for its enormous investment, amid speculation that it is heavily reliant on archive footage rather than new material. The show’s executive producer, Emma Cooper, recently left the independent production company, which is owned by Vice, for unknown reasons.

While the Netflix show is speculated to cost more than £1m for each hour-long episode, the 2017 BBC Panorama documentary Madeleine McCann: Ten Years On is understood to have cost less than £200,000. Netflix declined to comment.

One rival documentary producer said: “There’s been a lot of speculation about the series ever since it was first heard about, but particularly since the trailer was delayed.

“Around 50 people are thought to have been interviewed for it, so it’s taken a while to make and get everything signed off. Although there are eight episodes, it looks as though it is not going to be a big ‘reveal’ kind of show that some were expecting, more of a narrative piece, although apparently some people are talking for the first time in it.”

The McCanns have been offered the chance to view the Netflix documentary in advance, although they declined to do so. The family, who are known for keeping a close eye on claims made about them through their lawyers, Carter Ruck, are not thought to be considering any legal action at this stage.

However, media lawyers pointed out Netflix documentaries remain available to view for substantial periods of time, meaning the risk of libel damages could be higher than with a traditional one-off TV broadcast.