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The bill for the Northern line extension to Battersea Power Station is set to soar by as much as £240 million following “significant” design changes.

Transport for London and the Malaysian-owned developers of the former electricity generator are locked in talks about the overhaul of the plans for the Underground station at the £9 billion development — and who will pick up the extra costs.

The Standard understands that the changes relate to a section of the 39-acre site known as Prospect Place, a dramatic cluster of apartment buildings designed by the Californian “starchitect” Frank Gehry.

The new plans envisage more ambitious “over station development” (OSD) than in the original Battersea Power Station masterplan from Uruguayan architect Rafael Viñoly.

Now the Underground stop is to be fully incorporated into Prospect Place so that passengers arriving at the power station will emerge direct into one of its most striking buildings.

However, this will require the station “box” where the platforms and track are located underneath the building to be far stronger than planned in the original contract. The soaring costs were revealed in minutes of last month’s meeting of TfL’s policy and finance committee, which warned that the changes led to “a requirement for significant additional design work to develop a revised integrated station design which is capable of supporting the very different OSD to that originally intended and contracted”.

The Northern line “spur” serving the power station and Nine Elms is seen as vital to the success of the vast regeneration of the south bank district.

Battersea Power Station Development Company is already committed to making a £200 million contribution to the cost of the extension, initially estimated at up to £1 billion.

A £500 million contract was awarded to construction consortium Ferrovial Agroman Laing O’Rourke in 2014 and work was officially launched by former Mayor Boris Johnson in November.

David Hughes, director of major programme sponsorship at London Underground, said: “Changes have been made to the proposed development and the new Tube station beneath. These design changes will lead to an increase in the overall cost of the project.

“We are in constructive discussions with the developer about how this cost will be attributed. We are now working with Battersea Power Station to agree the necessary changes to the contractual arrangements between us to reflect these revised designs.”

It is understood the final estimate of the extra costs has not yet been determined but it could be as much as £240 million. A TfL spokesman said the changes will not affect the overall programme for the line, which is due to open in 2020.

A Battersea Power Station spokeswoman said: “Provision for design changes was made at the outset, given the scale and complexity of this major infrastructure project. TfL and BPS are working collaboratively on those designs and any related costs.”