The National Capital Authority has all but ruled out knocking down Commonwealth Avenue Bridge and replacing it.
Instead, the authority will focus on strengthening and upgrading the existing bridges.
It has submitted a draft business case on the proposed improvements to Infrastructure Australia, after completing surveying works.
The authority caught the ACT government by surprise last year when it announced it was investigating whether the bridge should be replaced all together.
The territory had been planning on building a third bridge in between the existing structures for light rail stage two to cross Lake Burley Griffin.
While the business case still includes a replacement scenario, the authority's chief executive Sally Barnes said it was an unlikely option.
"The notion of a new bridge that would carry light rail and traffic is probably off the cards," she said. "Never say never, but it's probably not feasible."
She said a replacement bridge was doubtful because it would be too costly and cause too much disruption.
"And the fact that we all love those existing bridges," Ms Barnes said.
The proposed work aims to increase the load-bearing capacity of the current bridge to handle current and expected future traffic, and widen the bridge to meet modern design standards.
It would aim to extend the design life by at least 50 years and would upgrade the vehicle and pedestrian safety barriers.
Transport Minister Chris Steel said the government's preferred option for light rail crossing the lake remained a third bridge in the middle of the existing structures.
"The decision is consistent with the federal government's response to the joint standing committee in 2018, which is consistent with the original visions for the bridge when it was built," he said.
Construction of the third bridge would not start until federal approval for light rail stage 2B was granted, Mr Steel said.
"This approval is subject to the environmental impact statement process," he said.
"The ACT government is continuing to work closely with the National Capital Authority in relation to the two projects.
"At this stage we are not aware of any particular issues with either project that could influence the sequencing of works."
The business case to strengthen Commonwealth Avenue Bridge was funded in last year's federal budget.
Ms Barnes said the work to upgrade the bridge could be done independently of the work the ACT government needs to do to allow light rail to cross the lake.
Last year's announcement by the authority took then transport minister Meegan Fitzharris by surprise.
"From our point of view there are some really critical aspects of that bridge whether or not we're building light rail and that's the heritage value of that bridge, the very important part the bridge plays in the parliamentary triangle," she said at the time.
"I think for the community of Canberra that bridge is a much loved bridge that the community would want to know and need a pretty good case for the replacement of."
Commonwealth Avenue Bridge opened in 1963, the fourth to be built in that location but the first to cross the new Lake Burley Griffin.
The previous three crossed the Molonglo River, before it was dammed to create the lake.
Then-prime minister Robert Menzies described the bridge as the "finest building in the capital" at its official opening.
The bridge also contains four granite stones from the original Waterloo Bridge in London that was built in 1817 before it was demolished to make way for a new one in the 1940s.