Variations in blood pressure were also important indicators of the risk of a heart attack, heart failure and chest pain, the findings show.
Experts from Oxford University, who carried out the research, called for treatment guidelines to be reviewed as such changes in blood pressure levels are currently ignored as irrelevant.
Prof Peter Rothwell, who led the study, said: “Previously, such fluctuations were considered to be benign and uninformative.
"We have shown that it is variations in people's blood pressure rather than the average level that predicts stroke most powerfully.
“Occasional high values, and what might be called episodic hypertension, carry a high risk of stroke.”
Patients whose “average” levels are considered normal but who suffer such spikes should be treated with blood pressure lowering drugs, he added.
There are an estimated 15 million people in Britain with high blood pressure and drugs to lower levels are among the most commonly prescribed medications on the NHS.
The research looked at how blood pressure variability affected stroke risk in four trials, each including more than 2,000 patients, all of whom had previously experienced a so-called "mini-stroke", or transient ischemic attack (TIA).
Those with the greatest variation in their systolic blood pressure – the pressure measured with each "surge" of the beating heart – were six times more likely to have a stroke than those with the least fluctuation, independent of their average readings.
Those whose 'swings' in blood pressure also peaked at high levels had a 15-fold increased risk, according to the findings, published in the Lancet medical journal.
Fluctuations were also a warning sign of heart failure, angina and heart attacks, independent of average blood pressure, the research found.
Two other studies, published in The Lancet and The Lancet Neurology, found that three groups of blood pressure drug, ACE inhibitors, angiotensin-2-receptor blockers and beta blockers, increased variation, while another two calcium channel blockers and non-loop diuretic drugs, reduced it.
Prof Rothwell said: "The work shows that many patients need blood-pressure-stabilising drugs – a new concept – as well as blood-pressure-lowering drugs."
Prof Peter Weissberg, from the British Heart Foundation (BHF), said: "High blood pressure is extremely common, and an important risk factor for heart attack and stroke. These researchers suggest that measuring blood pressure variability – rather than single recordings of blood pressure – may be important.
"Current practice is not wrong, but this might add a new measure to help doctors make decisions on who to treat for hypertension and which drug to use.
"People taking beta blockers should not be alarmed or stop their treatment suddenly, which can be dangerous. There are several potentially life-saving reasons to give someone a beta blocker. Those with concerns should talk to their GP."
Joe Korner, from the Stroke Association said: "These studies show that people whose high blood pressure varies, known as episodic hypertension, have a particularly high risk of stroke. This could have a huge impact on the future prevention of stroke because at the moment they are often not treated.”
Blood pressure is expressed using two measurements, systolic and diastolic.
Systolic, the first number in a blood pressure level, reflects pressure in the arteries as the heart beats, while the diastolic, the other number, is the pressure between beats.
Blood pressure around 120/80 is considered normal but most otherwise healthy people are not thought to suffer ill effects up to around between 130/80 and 140/90. A blood pressure level between 140/90 and 160/100 is known as mildly raised, while above 160/100 is considered too high.
Every year an estimated 150,000 people in Britain suffer a stroke.