HIV-related deaths could be reduced by 20% over the next five years if treatment begins earlier, the World Health Organisation (WHO) says in fresh guidance.
There were over 5m people receiving treatment for the virus at the end of 2009, up more than a million from 2008 - the largest ever increase in a year.
The guidelines would raise the number needing treatment to 15m by 2015.
But there are concerns about funding as countries potentially cut back on aid.
Unveiling its first new guidance for four years at the Aids 2010 conference in Vienna, the WHO says it wants treatment with a cocktail of drugs to begin before the virus seriously undermines the patient's immune system.
This means changing the threshold at which a drugs regimen should be started from 200 CD4 cells per microlitre of blood to 350, regardless of symptoms. These cells are the key marker of the health of the immune system.
Starting treatment sooner could prevent opportunistic infections such as tuberculosis, which is the number one killer of those with HIV.
"In addition to saving lives, earlier treatment also has prevention benefits," says Dr Gottfried Hirnschall WHO director of HIV/Aids.
"Because treatment reduces the level of virus in the body, it means HIV-positive people are less likely to pass the virus on to their partners."
Expanding treatment to more people will push the costs for 2010 up to $9bn, according to UN estimates. But experts stress that extra costs would be more than offset by decreased hospital costs, increased productivity, fewer children orphaned by Aids and a fall in new HIV infections.
"The investments we make today can not only save millions of lives but millions of dollars tomorrow," says Dr Bernhard Schwartlander of UNAIDS.
"People with weaker immune systems who come late for treatment require more complex and costly drugs and services than those who start treatment earlier and are healthier."
Front of the queue
The new threshold, which many doctors in richer countries already follow, is thought most likely to be applicable to those states with the resources and infrastructure for treating people earlier.
There remain more than 4m poor people whose infections are progressing and who cannot access treatment even under the old criteria.
The WHO says that despite its new guidance, the principle remains that those most in need of treatment should remain the priority.
More also needs to be done to encourage people to seek testing early, as many wait until symptoms appear when the viral load is already high.
The new drive takes place against a backdrop of uncertainty about the future of funding to fight the disease.
A report published by UNAIDS found that overall support for the global AIDS effort from donor nations flattened out last year in the midst of the global economic crisis.
The Global Fund to fight Aids, Tuberculosis and Malaria says it needs up to $20bn over the three years to maintain progress on tackling HIV infections.
The UK, for instance, last month put the fund and many other agencies on notice to prove that the spending works or face cuts.
The charity Medecins Sans Frontieres said there had been rapid progress in 2009 but it was concerned about the fate of 10m waiting to start treatment amid signs of a decreased commitment.
"Today international donors expect doctors to tell patients to come back for treatment when they're at death's door," said Dr Eric Goemaere, medical co-ordinator at MSF in South Africa.
"This is bad medicine. As a doctor I'd much rather give a patient pills today and send her home, than delay treatment and see her in six months at the hospital with complicated tuberculosis."