ITV has announced plans to close The London Studios, its TV production base on the South Bank, as a precursor to the development of a new headquarters building.
ITV's complex of studios and offices on Upper Ground was built in the early 1970s for London Weekend Television.
Some of the channel's best-known programmes including Good Morning Britain, This Morning and Loose Women are produced at the studios.
These daytime magazine shows will return to the site after redevelopment, but the larger studios currently used for other entertainment productions will not be replicated.
An ITV spokesman said: "After giving careful consideration to all London property options, we are planning to build a modern, purpose-built HQ on our existing South Bank site, bringing together all of our London-based staff in one location for the first time."
Gerry Morrissey, head of the BECTU union, said: "Today's announcement is a betrayal of dedicated staff at The London Studios who have worked to build a go-to production facility.
"Even in the short term the proposal has no logic. Why announce the studios closure when planning permission has yet to be sought and when the redevelopment will not begin until 2018 at the earliest?
"We'll be asking the questions which need to be asked to shine a light on the plans and to defend the interests of our members.
"We'd also like to know whether the proposal to close The London Studios is motivated more by the prospect of a possible sale of the company than by sound industrial planning."
Responding to BECTU's statement, the ITV spokesman added: "The proposed closure is a little over a year away, but we felt that it was important to let those affected by this plan hear about it first hand, and as soon as possible, which is why we're speaking to those colleagues today.
"We have a very clear business strategy: to make great content, showcase it on our channels and distribute it internationally, and we have given very careful consideration to what our redevelopment plans mean for The London Studios, which would require very significant investment to replicate within our redevelopment.
"We are therefore proposing to close the The London Studios business and use studio capacity in the external market, as we currently do for many programmes, to meet our future business needs."
Together with the adjacent Gabriel's Wharf, the land occupied by the London Studios is identified as a redevelopment site in Lambeth planning policy.
Amongst other criteria, Lambeth Council says it will welcome development of the site that "facilitates and responds positively to the proposed Garden Bridge".
Bishop's ward councillor Kevin Craig has welcomed news of ITV's long-term commitment to the South Bank.