William, now 35, and Harry, now 32, were just 15 and 12 respectively when Princess Diana died in a car crash in Paris in 1997. The pair walked as part of the funeral procession when the ceremony took place in September of the same year - an experience which William admits in the clip was 'one of the hardest things I've ever done.'
'I felt like she was almost walking along beside us, to get us through it,' he adds.
His brother Harry is also shown recalling the emotional day, referencing the moment when he and his family greeted the assembled crowds that had lined the streets outside Westminster Abbey.
'I remember people's hands were wet because of the tears that they'd just wiped away,' he says.
The documentary has been made to coincide with the 20th anniversary of the loss of Diana, and capture what the channel describes as 'one of the most extraordinary weeks in modern history.'
In another excerpt from the show released earlier this year, the princes described their grief and sorrow at being unable to 'protect' their mum, and explained that this was part of their motivation for wanting to participate in the candid interviews.
'Part of the reason why Harry and I want to do this is because we feel we owe it to her...' William detailed. 'I think an element of it is feeling like we let her down when we were younger.'
'We couldn't protect her. We feel we at least owe her 20 years on to stand up for her name and remind everybody of the character and person that she was. Do our duties as sons in protecting her.'
Diana, 7 Days airs on BBC One at 7:30pm on Sunday 27 August