Rustling gangs are stealing sheep dogs to help them take up to 100 animals a night during lucrative raids, the National Farmers Union (NFU) has warned.

Livestock worth £3million were stolen from UK farms last year, with large-scale sheep thefts fuelling the surge, according to rural insurer NFU Mutual.

It is now commonplace for 50 to 100 sheep to be stolen in a single raid by thieves who use working sheepdogs to round up large numbers to be driven away in the back of their lorries.

Once seen as an opportunistic crime, the NFU said that rustling gangs have become increasingly organised as they look to make huge profits from selling the animals on illegally.

Rustling has become the most costly crime for the UK's farming sector after agricultural vehicles and machinery theft, the latest NFU Mutual claims figures revealed.

It is estimated that a single sheep is worth £100, and a trained sheepdog around £1,500 on the black market.

The NFU dealt with a recent case of a working sheep dog, stolen from Leicestershire, that was worth between £4,000 and £5,000.