A third of stroke patients are still not treated in a specialist unit despite improvements in access, a national audit has shown.
Fast diagnostic scans are key
And fewer than half of patients receive brain imaging within 24 hours, a figure experts said was "unacceptably low".
Welsh patients fare the worst with only nine specialist units in the country, the Royal College of Physicians found.
The government said it would publish a National Stroke Strategy this summer to "accelerate" the rate of improvement.
Patients who receive care in a specialist unit have a 50% reduced risk of death and better long-term recovery.
National guidelines published in 2001 recommend all patients are treated in a specialist unit but similar guidelines were not introduced in Wales until 2006.
The College said there had been significant improvements but some hospitals had failed to recognise that stroke patients needed "21st century care".
The audit, funded by the Healthcare Commission, found the number of eligible hospitals in England with stroke units increased from 82% in 2004 to 97% in 2006.
But only 28% of patients in Wales were treated in a stroke unit compared with 64% in England and 73% in Northern Ireland.
Only 15% of patients are admitted to a stroke unit on the day they arrive at hospital.
And although brain scans are needed to determine treatment, patients admitted on a weekend often have to wait until the next working day.
Those with minor strokes who are in hospital for less than two days are least likely to have access to specialist services.
Dr Tony Rudd, chair of the Intercollegiate Stroke Network, said more than 90% of patients should be treated in stroke units.
"And not to have done imaging by 24 hours is not really good enough. We need to differentiate between bleeds and blocked arteries."
He added: "The failure of the majority of hospitals in Wales to offer stroke unit care is scandalous and needs urgent action"
Dr Rudd, who is also consultant in stroke medicine at St Thomas' Hospital in London, said the National Service Framework in the UK had successfully increased the number of specialist units but there still wasn't the capacity to deal with every patient.
Dr Jonathan Boyce, head of clinical audit at the Healthcare Commission, said the study showed welcome improvements.
"But there is still too much variation, too many places and regions that are not responding as well as they could to minimise the harm done by this serious and common condition.
"They now need to get their house in order."
Joe Korner, director of communications for The Stroke Association, added: "Stroke units can halve your chance of dying from a stroke, so it is a scandal that getting treated on one is a matter of luck or your postcode."
A spokesperson for Health and Social Services at the Welsh Assembly said the report showed stroke services in Wales needed improvement but there was a strategy in place.
"Over the last year, NHS organisations in Wales have been required to develop more appropriate models of care for patients suffering with strokes.
"Officials at the Welsh Assembly Government will work with the NHS to develop clear national priorities for action."
Professor Roger Boyle, national clinical director for stroke and heart disease, said the pace of improvement needed to accelerate.
He said a national stroke strategy would be published for consultation this summer which will include a toolkit to help hospitals commission stroke services.
He added: "I would also urge the NHS to use this report as a tool to examine how their stroke services."