The cases were confirmed by the UK Health Security Agency (formerly Public Health England) who are supporting the nursery with the outbreak.
It said the infected children are receiving treatment and recovering well, whilst a number of control measures are now in place to control the outbreak and prevent further spread.
The UKHSA added that it is undertaking a “detailed risk assessment in line with national guidance”, but did not say whether the nursery has had to temporarily shut as a result of the cases.
It did not say which nursery is affected, but it is asking parents in Lancashire to be aware of the symptoms and take measures to prevent any cases spreading.
A spokesman for UKHSA said: “Our North West Health Protection Team is supporting with an outbreak of scarlet fever at a Lancashire nursery.
"Scarlet fever is a contagious infection that mostly affects young children. It's easily treated with antibiotics.
“The first signs of scarlet fever can be flu-like symptoms, including a high temperature, a sore throat and swollen neck glands (a large lump on the side of your neck).
“A rash appears 12 to 48 hours later. It looks like small, raised bumps and starts on the chest and tummy, then spreads. The rash makes your skin feel rough, like sandpaper.”
Dr Merav Kliner, regional deputy director for UKHSA, added: “It’s not uncommon to see a rise in cases of Scarlet fever at this time of year and we are continuing to monitor rates of infection across the North West.
"Scarlet fever is highly contagious but not usually serious and is easily treatable with antibiotics to reduce the risk of complications and spread to others.
"It is important to take antibiotics, as instructed by your GP, to minimise the risk of complications.
"The UKHSA reminds parents to be aware of the symptoms of Scarlet fever and to call their GP or NHS 111 for further advice or assessment if they think their child might have it.
"To limit the spread of Scarlet fever it is important to practice good hygiene by washing hands with warm water and soap, not sharing drinking glasses or utensils, and covering the nose and mouth when coughing or sneezing.”