Australian firm Freehill Hollingdale & Page is closing its three-lawyer London office, although the other five Australian firms say they will stay.
Kevin Lewis, London partner at Freehills, said his term of duty was coming to an end and the firm had decided to review whether it was worth staying.
"When we decided to open here in 1989 our competitors were here and we were worried about the effect that might have on our client base back home. But they are not doing too much that would affect us.
"We were also nervous about what was going to happen in 1992 when EU border controls were removed. That proved to be a non-event."
He said his clients already worked with partners in Australia and the London office had been largely a matter of convenience for them.
London's five other Australian firms said they intended to stay, although several said work had declined in the past few years, with more Australian firms chasing fewer clients.
Justin Smith, resident partner at Blake Dawson Waldren, which opened in London in 1991, said: "There may be an argument that none of us should have opened up here. We are used by clients for convenience more than anything, largely because of the time differences with Australia. But now we are here there is a need to continue servicing those clients."
Michael Whalley, who set up Minter Ellison's London office in 1974, said: "With six firms chasing the same cake everyone's got a thinner slice."
The other three firms are Allens Arthur Robinson, Mallesons Stephen Jaques and Corrs Chambers Westgarth.