A BBC spokesperson said this week it would decide whether Schal or Bovis Lend Lease would get the job in the next two months. A decision was originally expected in July.
The spokesperson said: "We are currently negotiating the completion of a planning agreement. We are on target to commence work in the autumn, so the decision will be made by then."
The BBC added that it was still in negotiations with Land Securities Trillium over which developer is to take over the project. The firm owns the rest of the BBC's London property portfolio and is expected to take over the Broadcasting House redevelopment.
Sources close to the bidders claimed these talks were slowing the decision on the construction manager down. Land Securities was due to submit a business case for the scheme this week. One source said: "The BBC is still weighing up its options. I don't think it's got the price it wants for this job."
The BBC may still decide to manage the scheme in-house and sources claim that Schal owner Carillion is offering the corporation a PFI-style funding package for the job. One said: "The Land Securities deal is still plan A, but the BBC is looking at other options."
The talks came as it emerged that consultant Ernst & Young had been brought in to assess the progress of the MacCormac Jamieson Prichard-designed scheme.
The consultant is understood to have written a value-for-money report on the project. The BBC declined to comment on such a report. A spokesperson said: "It's a hugely complex project which needs many professional disciplines. Ernst & Young is a member of the team."
A source close to the project said it was proving sensitive. The source said: "It is subject to an enormous amount of consultation right now."
Bovis Lend Lease and Schal emerged from a shortlist that included Mace, Heery International and Amec. The scheme received planning permission in June.
Bidders for the scheme raised concern earlier this year that Bovis Lend Lease would be favourite for the job because of its links with Land Securities. Bovis is building the £200m White City BBC centre for the developer.
The newly developed Portland Place will be home to BBC Radio and Music, BBC World Service and BBC News. It will include a public arcade linking Portland Place and Hallam Street.