Officials say they were responsible for transporting billions of pounds of oil and oil products in the last year.
The sanctions are the largest UK package of its kind, the Foreign Office said.
Half of the ships transported more than 4.3 billion dollars (£3.4 billion) of oil and oil products in the last 12 months, and two insurance companies have also been targeted with sanctions.
The shadow fleet is made up of ships engaged in illegal operations to evade sanctions.
Speaking at the G7 foreign ministers’ meeting in Italy, Foreign Secretary David Lammy told reporters: “This morning I am announcing the biggest sanctions package of the Russian shadow fleet and it underlines the UK commitment to Ukraine.
“We are supporting them by £3 billion a year every year for as long as it lasts to support them militarily, and of course with both economic and humanitarian aid we continue to support Ukraine.
“It’s hugely important at this G7 that all colleagues across the G7 continue to stand with Ukraine for as long as it lasts and we are confident that Ukraine can have the funds and the military equipment and kit to get through 2025.
“Our assessment is that (Russian President Vladimir) Putin shows no signs at all of wanting a negotiation. We have to ensure that Ukrainians are equipped for the fight.”
Asked for more detail about the sanctions, Mr Lammy added: “We are shadowing 30 Russian vessels. We are determined to ensure that both the ships, the enablers of those ships thwarting European and UK sanctions, are hurt at this time and that we ensure that the attempts to circumvent the sanctions that we put in place previously and of course to avoid Russia using its oil revenues to continue to fund its war – that we bear down on those issues.”
During the G7 summit this week, Mr Lammy is expected to discuss the conflict with his counterparts from other nations, and to encourage them to keep up with efforts to support Ukraine, amid concerns over what the re-election of Donald Trump as US president could mean for support for Kyiv.
Cabinet minister Pat McFadden is expected to say that British spies will seek to counter the threat of Russian cyber warfare with a new laboratory for artificial intelligence.
The Chancellor of the Duchy of Lancaster will say the UK and its Nato allies are “watching” Moscow and combatting its attacks both publicly and “behind the scenes” as he delivers a major speech at the Nato cyber defence conference on Monday.
