Camille Coduri, who played Jackie Tyler in Doctor Who, and Danny Webb, best known for his role in Alien 3, were both scheduled to appear at NOR-CON, which returns to the Norfolk Showground on Saturday, September 26 and Sunday, September 27.
Ms Coduri has pulled out for health reasons, while Mr Webb cancelled due to “circumstances beyond our control,” according to organisers.
Despite the cancellations, several other guests remain on the line-up and it understood replacement guests are being sought.
Confirmed guests for the weekend include former Gladiator Jamie Bigg, Star Wars actor Ross Beadman, and Sophie Aldred, who played Ace in Doctor Who.
Comic artists and authors will also be in attendance, including Jay Manchand, an officially licensed artist for Star Wars, Game of Thrones, and Stranger Things.
A full orchestra will perform TV and film themes free of charge on Sunday, featuring classics such as The Imperial March from Star Wars and Superman’s Theme.
Fans can also enjoy a wide range of vehicles, props, and displays, including the UK Garrison of the 501st Legion and an Optimus Prime rig photo opportunity.
A spokesman for NOR-CON said: “Whilst we’re disappointed to announce that Camille and Danny are now unable to attend this weekend, these things are out of anyone’s control and they do happen. We wish Camille and Danny all the very best, and look forward to welcoming them to NOR-CON another year. They’ll always be welcome here and we’d be delighted to have them back.
“We are, however, really pleased to have secured three more guests that will be attending NOR-CON.
“We’ve worked really hard to ensure that people won’t miss out, so having Catrin Stewart, Adam Brown and Daniel Anthony coming to Norfolk to celebrate our 15th anniversary is incredible.
“Catrin is best known for her much-loved role as Jenny Flint in Doctor Who, as part of the Paternoster Gang with Madame Vastra and Strax the Sontaran.
“Adam played Ori the dwarf in The Hobbit, featured as Jack Sparrow’s new recruit Cremble in Pirates of the Carribbean: Salazar’s Revenge, and also supported Rhaenyra Targaryen as Lord Glendon Footly, head of House Footly, in House of the Dragon.
“Anyone who grew up watching The Sarah Jane Adventures on CBBC will recognise Daniel, who played Clyde, one of the main characters in Sarah Jane’s entourage, and was also nominated for a British Soap Award for Best Newcomer in 2006 for his role in Doctors.
“These three are amazing actors, and they really speak to what we’re doing at NOR-CON to make our 15th anniversary the best yet.”