The UK tax authorities (HMRC) announced on June 14, 2019 that supply of renewable energy certificates would be subject to a reverse charge "with immediate effect." Read the full Explanatory Memorandum here.
The energy certificates, sold on exchanges, may have been being used in missing trader intracommunity (MTIC) frauds. MTIC frauds turn on the differences in the way value added tax (VAT) is treated in different states within a community, such as the European Union. Typically, when a good subject to a VAT is exported from one state to another within a community the exporter is refunded for all VAT paid to that point. The idea is that the good is being exported and will be "consumed" elsewhere -- so the VAT, a consumption tax, should not be collected in the exporting state. The importer is then tasked with voluntarily remitting payment for their import in their state. An MTIC fraud takes advantage of this by, more or less, simply not having the importer remit payment -- also called a reverse charge. This can be an intractable problem when the taxing authority of an importing state has no way of knowing the good had been imported to begin with.
MTIC frauds can be more sophisticated, with the same good being sent back to the export country and cycled around again for another round of VAT refunds -- a so called "carousel" fraud.
Here, HMRC acted quickly to close the loophole that allowed this renewable energy certificate scam to continue -- citing a "serious and credible threat to the VAT system." Without more information it is difficult to know exactly what the scam was, but if the fix is forcing a reverse charge on the supplier (in the above simplified example, the exporter), then the fraud was likely a missing trader of some variant.