West Ham and their London Stadium landlords are involved in a legal dispute over plans for 3,000 additional seats.
The Hammers are the lead tenants of the former Olympic Stadium on a £2.5 million-per-year 99-year lease.
They want to add 9,000 seats to take the capacity of the arena to 66,000, Gerry Murphy, the acting chief executive of the London Legacy Development Corporation (LLDC), told the London Assembly on Wednesday.
West Ham say their first plan is to add 3,000 seats to their 57,000 capacity "to initially reach a capacity of 60,000, as was promised publicly to our supporters by all London Stadium stakeholders."
The dispute with the LLDC centres on the operating costs of making the additional seats available for use -- they are already in the arena -- plus the revenue from those seats if they are made available.
Murphy told a London Assembly Budget Monitoring Sub-Committee meeting: "There is a dispute with West Ham about what the contract says in terms of capacity.
"West Ham have absolutely said that they want to increase the capacity to 60,000, and actually they want to further increase the capacity to 66,000.
"LLDC and E20 [the landlords] disagrees with their interpretation of the contract.
"We feel that actually if West Ham want to enjoy more seats then they should commensurately pay more.
"And that's the gist of the disagreement between us and it is subject to legal proceedings."
West Ham say they are already paying for a 60,000-seat stadium -- the capacity the club announced in March 2016.
A West Ham spokesperson told Press Association Sport: "These seats are in London Stadium already, have always been there, and have already been paid for under the terms of our concession agreement.
"Having sought a resolution of this matter for many months, West Ham United are seeking a legal declaration."
Murphy said West Ham began to ask for the increased capacity at the start of this season, their second at the venue since leaving Upton Park in May 2016.
Asked who would benefit from the revenue from any additional seating, Murphy said: "West Ham's argument is they would receive all of the extra revenue from the extra seats. We would contend we should get a share of that."
West Ham say all on the site would benefit from additional supporters attending matches.
LLDC receives a flat fee from the tenancy agreement, plus a share of the catering revenue and Murphy admitted that, under the current agreement made by her predecessors, LLDC makes an ongoing loss.
"The matchday costs will continue at some level to exceed the revenue that we get," she said.
Wednesday's meeting followed the publication of the Moore Stephens Olympic Stadium Review, which reported in November on the arena built for the London 2012 Olympic and Paralympic Games and funded by taxpayers.
London Mayor Sadiq Khan is to take over control of the London Stadium after an independent report said there had been "a catalogue of errors" leading to massively increased costs over the stadium's conversion.