AN investigation is being carried out at HMP Dartmoor, after three prisoners were taken to hospital 'at varying levels of consciousness' on so-called legal highs.

A fleet of seven ambulances was called to the prison in Princetown on Saturday at about 5.35pm, after a number of inmates became unwell.

Three were subsequently taken to Derriford Hospital in Plymouth.

In a statement, the Ministry of Justice said: 'A small number of prisoners received care in hospital after ingesting a substance believed to be a legal high.

'All are now in back in prison.'

It said legal highs were classed as contraband and, as such, were not allowed in prison.

A prison spokesman said: 'Staff at HMP Dartmoor take the use of illicit substances in prison extremely seriously and use a range of robust measures to keep them out, including specially trained dogs, random drug tests and searches.

'Anyone caught with such substances will be dealt with severely and can be referred to the police for prosecution.'

Designed to mimic the effects of illegal drugs, legal highs are on sale on websites and some high streets and are chemically different enough to not be covered by law.

MPs have expressed concern about prisoners' access to dangerous substances following last weekend's incident.

Alison Seabeck, MP for Plymouth Moor View, has called on the prison service to launch an investigation, while Garry Streeter, MP for South West Devon, said: 'I consistently fail to understand why we cannot stop drugs being smuggled into prisons.'