Mr Blair said NHS Direct now covered nearly two-thirds of the population of England - about 32 million people - and was on course to reach everyone by December 2000.
The scheme was launched last year as one of the Government's flagship schemes to ease pressure on GPs and hospitals by offering round-the-clock nursing advice via a nationwide helpline.
Although it was initially available to only 40 per cent of the population, its 17 call centres have spread the service and 10 more should open by next April, Mr Blair said.
The Net site will provide on-line information on more than 180 conditions and advice on smoking, drinking and diet.
The new NHS Direct Healthcare Guide lists the 20 most common health symptoms and offers advice on how to treat them at home without resorting to the overstretched 999 or family doctor services.
Anyone still worried can talk to a nurse on the 24-hour phone line for the price of a local call. In some areas, the public can use the service to arrange a GP visit outside normal surgery hours.
The guide, which includes advice on symptoms such as chest pains, fevers and nausea, gives step-by-step help on the self-treatment of minor complaints. More than 1.2 million copies of the book will be made available in 8,000 pharmacies this week. All new mothers will be given a copy.
Launching the new service in Bootle, Merseyside, the Prime Minister said the publication of the Healthcare Guide was the first time any national government in the world had produced a specialist "self-help" handbook linked to a dedicated round-the-clock helpline staffed by professional nurses.
Mr Blair said the NHS Direct service had already taken nearly one million calls. Research showed that 97 per cent of callers were satisfied with the service they received and understood the advice given. In nearly two-thirds of cases, patients were advised to take a different course of action than the one they had intended.
"NHS Direct is extending care right into the home, helping people to take more responsibility for their own health and helping parents make the right decisions about care for their children," Mr Blair said. "There are few more worrying situations than for a parent to wake up to a crying and sick baby in the middle of the night. You don't know whether to bother the doctor, go to casualty or wait till morning. To have NHS Direct to call on provides massive and welcome reassurance."
Mr Blair said the service had already saved lives - and he met James Greenwood, a teenager whose serious heart condition was spotted by an NHS Direct phone nurse.
Mr Blair said he hoped that by the end of next year the service would be the nation's main "gateway" to out-of-hours care. He added that, with the growing impact of the Internet, an online version of the service would be invaluable in giving information on medical advice and facts. In time, the online service could become one of the most popular Internet sites, and could even offer an interactive service linking patients with specialists or other patients to discuss particular conditions.