Britain's spy agencies will reveal its knowledge of alleged Westminster-related child abuse at a public inquiry amid concerns it aided in an establishment cover-up.

MI5, MI6 and GCHQ have given their "full cooperation" with the Independent Inquiry into Child Sexual Abuse, lead counsel Andrew O'Connor QC told a preliminary hearing on Tuesday.

However, it emerged yesterday that some of the evidence the agencies will give may be heard in private due to national security reasons.

Mr O'Connor said: "We have been asked expressly on behalf of several of the core participants [of the inquiry] whether all of the evidence from the security and intelligence agencies will be given in public hearings, or whether it will be necessary for some of their evidence to be adduced in a closed hearing -- in other words, in secret.

"The short answer to this is that the position is not yet clear. We very much hope that all the agencies' evidence will be given in public hearings.

"We know from our discussions with them that the agencies recognise both the importance of this investigation and the need for its work to be as transparent as possible.

"They have told us that they wish to be as open as possible. However, it is, of course, inherent in the work that the agencies do that some information -- about, for example, records that they do or do not hold -- cannot be given publicly without damaging national security interests."