Britain says the "shadow fleet" uses illicit practices to avoid Western restrictions on Russian oil.
The government said it was working with maritime authorities to demand that Russian vessels with suspected dubious insurance provide details of their insurance status as they pass through the English Channel.
"Any actor that facilitates and supports Russia's malign activities could be exposing themselves to sanctions," the government said.
Russia rejects Western pressure to limit its oil exports, and in the past year, there has been growth in the number of tankers transporting cargoes that are not regulated or insured by conventional Western providers.
The 18 oil tankers will be barred from UK ports and unable to access British maritime services, bringing the total number of sanctioned Russian oil tankers to 43.
The vessels included NS Bora, Atlas and Moskovsky Prospect.
"The UK's relentless action against the shadow fleet is putting grit into the system and starving (Russian President Vladimir) Putin's war machine of crucial revenues," the government said.
The targeted oil tankers transported an estimated 4.9 billion pounds ($6.37 billion) worth of oil last year, it said.
As part of Thursday's action, the UK also sanctioned Russian gas company Rusgazdobycha JSC.
"We must combat malign Russian activity at every turn, whether illicit tactics to bolster Putin’s war chest, their use of cyber-attacks or barbarism on the front line in Ukraine," Foreign Secretary David Lammy said in a statement.
