A colony of one of the UK's rarest butterflies has grown by 10 times in the past year, the RSPB has said.

There were about 120 heath fritillary butterflies at Blean Woods National Nature Reserve in Kent in 2008 - and there are about 1,300 this year.

Michael Walter, manager of the reserve, said: "The sheer number of butterflies is unlike anything I've seen before for any UK butterfly species."

The colony numbered just six in 2006, at the reserve near

The fact that they are heath fritillaries makes it even more incredible."

He added: "I have been working at Blean Woods for 27 years and have seen some amazing sights but this has got to be one of the best.

"Wherever I look there are just great clouds of them taking off.

"In the evening you can see them settle on the leaves here, they seem to cover every surface."

The large colony is one of 21 groups of heath fritillary at the reserve.

The caterpillars feed only on cow-wheat plants.