First this, from a Labour source:
Boris Johnson said he would be open and accountable but he is hiding behind the strictest possible interpretation of the rules to avoid disclosure. Given that a multimillion-pound development including affordable housing is involved, Johnson should act in the interests of transparency and release this information.
Then this, from a source close to the mayor’s office:
We are considering whether the Freedom of Information [FoI] request falls under the exemption for communications with Her Majesty and with other Members of the Royal Household. The FoI Act allows public authorities to extend the 20-day limit for responding to FoI requests to allow it further time to decide whether an exemption applies. In this case we’ve said we’ll get back by 24 February but not refused the request for details of correspondence between the Mayor and Prince Charles.
So, to be fair, the letters have yet to be blocked. It remains to be seen, however, whether they will be released.