A spokesman for Mr Cunliffe said he had not forgotten about it but the initial legal advice was that it did not need to be declared.
Mr Cunliffe sought advice from the Registrar of Pecuniary Interest, who advised that if there was any doubt it should be declared.
The register requires MPs to disclose all trust in which they have a beneficial interest or for which they act as a trustee.
Mr Cunliffe's initial legal advice is understood to have come from Greg Presland, who also acted as the trustee of Mr Cunliffe's Labour leadership campaign trust last year.
Mr Cunliffe referred to the trust earlier this week while he was answering questions about his use of trust for campaign funding.
He said the only other trust he had a beneficial interest in was the Bozzie Trust which owns his family home. The home was owned by a trust because his wife was a lawyer and the trust was for liability reasons.
Read more: Bryce Edward's round-up of the harsh scrutiny Cunliffe and the Labour party