The highly accurate test looks for biomarkers released by mast cells, white blood cells of a specific type that form part of the immune system.
In a study involving 174 children the Mast Activation Test (MAT) made a correct diagnosis 98% of the time.
Currently peanut allergy is confirmed using an unreliable skin-prick test and "oral food challenge" (OFC) - a time-consuming process that involves feeding the patient increasingly larger doses of peanut.
The new test is five times cheaper to carry out than an OFC. It could act as a second-line tool when skin-prick tests are inconclusive before referring children for an OFC, say the researchers.
Study leader Dr Alexandra Santos, a Medical Research Council scientist at King's College London, said: "The current tests are not ideal. If we relied on them alone, we'd be over diagnosing food allergies. Only 22% of school-aged children in the UK with a positive test to peanuts are actually allergic when they're fed the food in a monitored setting.
"The new test is specific in confirming the diagnosis so when it's positive, we can be very sure it means allergy. We would reduce by two-thirds the number of expensive, stressful oral food challenges conducted, as well as saving children from experiencing allergic reactions."
Peanut allergies are among the most common food allergies in children.
Around 5% to 8% of UK children have a food allergy, with up to one in 55 being allergic to peanuts.
Food allergy symptoms, such as itching skin, stomach cramps and constriction of the throat and airways, are triggered when food proteins interact with an antibody called immunoglobulin E (IgE).
Standard skin-prick tests measure the presence of IgE antibodies.
In contrast, the new test focuses on mast cells which are activated by IgE in blood plasma. They produce biomarkers associated with allergic reactions which can be detected in the lab.
The scientists are already looking at expanding the work to provide a way of diagnosing other food allergies.
Dr Santos said: "We are adapting this test to other foods, such as milk, eggs, sesame and tree nuts.
"This test will be useful as we are seeing more and more children who have never been exposed to these foods because they have severe eczema or have siblings with allergies. Parents are often afraid to feed them a food that is known to cause allergic reactions."
The study is reported in the Journal of Allergy and Clinical Immunology.