Russian gas producer Gazprom (GAZP.MM) under a long-term supply deal with Budapest that took effect on Friday is set to supply Hungary with gas via Serbia and Austria instead of through Ukraine.
FGSZ confirmed that it had launched the new interconnector pipeline from Serbia on Oct 1 in a reply to questions from Reuters.
"If a (trading) company gets hold of gas via Ukraine, it can continue to use the pipeline that comes from Ukraine and current shipments data show that there is a similar amount of gas coming from Ukraine today as via the new pipeline from Serbia," FGSZ said.
It said as a transmission system operator it could not disclose the identity of the trading firm involved.
FGSZ said dozens of trading firms have been shipping gas to Hungary besides Gazprom on various routes, one of which was the Ukrainian pipeline.
Sergiy Makogon, head of Ukraine's gas transit operator said in a Facebook post that Gazprom had not resumed gas transit through Ukraine to Hungary.
"There is currently a small flow of gas from Ukraine to Hungary, but this is not transit, it is the re-export of gas by European traders which they kept in Ukrainian underground storage."
Gazprom's decision to send gas via Serbia and Austria instead of Ukraine deprives Kyiv of transit revenue and also means it can no longer import reverse flow gas via Hungary, which it has been doing since 2015 as a way of not buying gas directly from Russia.
The dispute over Hungary's new gas deal with Gazprom has spilled into a bilateral dispute between Kyiv and Budapest, which are already at odds over the use of the Hungarian language in Ukrainian schools.
Britain's Queen Elizabeth carried out her first official engagement on Tuesday since spending a night in hospital and being ordered to rest by her doctors.