People suffering from cancer, multiple sclerosis and HIV will be legally protected from discrimination under a new law that comes into effect today.
Previously, they were only covered by the Disability Discrimination Act 1995 (DDA) if their condition had a long-term adverse affect on their ability to carry out day-to-day duties.
However, amendments to the act, which come into force today, now protect such people against discrimination in the workplace, education, housing or in accessing services from the moment they are diagnosed.
Announcing the news, work and pensions secretary John Hutton said the extension of DDA protection showed tackling discrimination was "right at the heart of the government's social agenda".
"We have worked closely with disability organisations and across government to bring about change in legislation to break down social and institutional barriers so people can enjoy fuller, enforceable right," he said.
"This is an important step in the government's ongoing commitment to end discrimination."
He added further changes to the law would be introduced next year which would require the public sector to involve disabled people at every step of their policy-making processes.
Other changes to the existing law will mean people with mental illness will no longer have to prove their condition is clinically well-recognised in order to be covered by the DDA, and local authorities and private members' clubs will be legally obliged to treat disabled people equally.
The news has been welcomed by disability rights campaigners and health groups, who say the new changes will improve access to justice.
Bert Massie, chairman of the Disability Rights Commission (DRC), said: "We have been unable to help people who have been diagnosed with serious illnesses and then treated unfairly, because they didn't fall under the legal definition of disability. This is plainly wrong.
"This closes a significant loophole in the law."
Today's changes come hot on the heels of the government's launch of a new Office for Disability Issues (ODI) last week, which will aim to give disabled people a stronger voice in policy-making.