West Ham do not have the power to prevent Tottenham Hotspur or Chelsea from sharing the Olympic Stadium, the London Legacy Development Corporation told a hearing on Monday.
The Hammers will move into the Olympic Stadium this summer and the club has previously said it would block any attempt by a rival to temporarily share the Stratford ground.
Tottenham, beaten to the Olympic Stadium by West Ham in 2011, are looking for a temporary home for the 2017-18 season while their new stadium is built next to White Hart Lane, and Chelsea will need a temporary home for at least two seasons during the redevelopment of Stamford Bridge.
"No one has asked us for our permission [to ground-share] and if they did we would probably say no, depending on who it is -- if you get my drift," West Ham vice-chairman Karren Brady said in 2014. "We are the anchor tenant for the winter matches and nothing else can happen in that time without our permission. Our football matches take priority over everything else."
However, at Monday's appeal hearing against the decision to release the full details of West Ham's deal with stadium bosses, the LLDC revealed the East London club do not have any power of veto, leaving Spurs or Chelsea free to pursue the option of a temporary ground-share at the Olympic Stadium.
But the LLDC also confirmed any ground-share would require the "co-operation" of West Ham, with a Hammers spokesperson adding it would be "impossible" to accommodate another club with their permission.
"As anchor tenant we have primacy of use during the football season and our contract gives us overriding priority to use the stadium, ensuring our fixtures and events are ring-fenced and will always take priority over all other events," a West Ham spokesperson said.
"It would therefore be impossible to accommodate the fixtures of another Premier League club without West Ham agreeing -- a position which was fully supported at today's hearing."
Wembley remains a more likely option for Tottenham and Chelsea -- with Spurs also considering stadium:mk, the home of Milton Keynes Dons -- and sources have told ESPN FC that the FA is already making preparations for the national stadium to host Premier League football in the future.