"We fully support Bulgaria's efforts in disrupting an alleged spy ring and taking steps to tackle Russia's hostile actions in its territory," Britain's Foreign Secretary Dominic Raab said on Twitter on Saturday.
Foreign direct investment, much needed to boost growth in the European Union's poorest member state, was 32.7 million euros in January, down from 108.4 million euros a year ago.
Bulgaria earlier this month expelled the Russian military attache after prosecutors opened a probe against him for spying on US troops based in the country.
He has previously said Bulgaria expects to start inoculations a week after the first 125,000 shots of Pfizer's COVID-19 vaccine arrive around the New Year.