Of the six main players in the UK energy market only Centrica-owned British Gas and Scottish Southern Energy have lowered their prices since the start of the year.
SNP energy spokesman Mike Weir said: "We want to know why their prices are not falling in line with wholesale price cuts and what they are going to do to bring them down.
"We also want to know about what investments they are planning on new generation power plants."
British Gas was the first energy firm to reduce its tariffs, knocking 10 per cent off its standard tariffs and nine per cent off its zero carbon deal.
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