The new system, which will be published in the Annals of the Missouri Botanical Garden next month, throws up some surprises.

For example, botanists used to think the papaya tree was a close relative of the passion flower. Now it turns out the papaya is actually a closer cousin of the cabbage.

The reclassification project is the work of nearly 100 scientists led by Mark Chase from the Royal Botanic Gardens at Kew in west London and two colleagues, Kare Bremer of the University of Uppsala in Sweden, and Peter Stevens of Harvard.

It has taken seven years to compete.

New techniques

It was not until the end of the eighteenth century that the Swedish naturalist Carl von Linné, known as Linnaeus, attempted to make a complete and systematic classification of plant families.

From his lead, botanists have since grouped flowering plants into the different families according to similarities in their shapes and structures - the number of petals, the shapes of their leaves and so on.

But before the technology was developed to sequence genes in an organism, establishing the true evolutionary relationships between plants was mere guesswork.

Now, by applying the new techniques of molecular biology, a more accurate picture has emerged.

Roses, for example, are more closely related to nettles than previously thought.