The government is investigating 37 UK-linked businesses for potentially breaking Russian oil sanctions - but no fines have been handed out so far, the BBC can reveal.

Financial sanctions on Russia were introduced by the UK and other Western countries following the invasion of Ukraine in 2022.

Conservative shadow foreign office minister Dame Harriett Baldwin said sanctions were designed to “shut down the sources of finance for Russia's war machine” and “bring this illegal invasion to an end sooner”.

But critics have claimed they are ineffective after the latest figures showed the Russian economy was growing.

The Treasury said it would take action where appropriate, but pointed to the complexity of the cases as a reason they take considerable time.

The sanctions include a cap on the price of Russian oil, designed to ensure that oil can keep flowing without Russia making large profits.

The cap prohibits British businesses from facilitating the transportation of Russian oil sold above $60 a barrel.

Data obtained by the BBC using Freedom of Information laws shows the Treasury has opened investigations into 52 companies with a connection to the UK suspected of breaching the price cap since December 2022.

As of August, 37 of those investigations were live and 15 had concluded, but no fines had been handed out.

The identities of the businesses are unknown but it’s understood some are likely to be maritime insurance firms.

Dame Harriett told the BBC “there is probably more that could be done” by the government and the oil sector itself “because it does appear that UK importers are still bringing in oil that originated in Russia”.

The anti-corruption organisation Global Witness said it was “quite astonishing” that no fines have yet been handed out, and described the oil cap as “a sort of paper tiger” that is failing to crack down on rule breaking.

Louis Wilson, the head of fossil fuel investigations at Global Witness, called for “bold action” to be taken against companies breaching sanctions.

He said if the UK government “prevents British businesses from enabling Putin's profiteering, then I think you'll start to see others following that lead”.

Investigations into potential breaches of the oil cap and other financial sanctions are carried out by a Treasury unit called the Office of Financial Sanctions Implementation (OFSI).

OFSI received an extra £50m of funding in March to improve enforcement of the UK’s sanctions regime

But Mr Wilson said companies under investigation find it “pretty easy to come by” a document that gets them out of trouble.

He described the documents as “basically promises, voluntary bits of paper” and said they can be easily obtained even if the company was involved in transporting oil sold above the price cap.

“What's likely is either these businesses will find the paperwork that they need to get through this process, or we'll see the UK government drop these cases quietly,” he said.

He claimed the US were reluctant to make the Western sanctions regimes harder “because they're scared that if they do enforce the rules it will stop the Russian oil trade and that will send oil prices higher”.

Dame Harriett said it was important that when OFSI “find deliberate wrongdoing they are exacting financial penalties”.

A spokesperson for the Treasury said it would take enforcement action "where appropriate" and it was "putting sanction breachers on notice".

They added that the cap was reducing Russia's tax revenues from oil, adding that data from the country's own finance ministry showed a 30% drop last year compared to 2022.

The former chair of Parliament’s Treasury Select Committee launched an inquiry into the effectiveness of sanctions on Russia in February.

Dame Harriett said she “received evidence that the oil price cap is being evaded by refining Russian oil in refineries based in third countries and then the oil is being exported into the UK.”

Earlier this year the BBC reported on claims about how much oil this so-called “loophole” is allowing into the UK.

But parliamentary committees are disbanded once an election is called and the findings of the Treasury committee inquiry were never published.

It’s understood no decision has yet been made as to whether the new Treasury Select Committee will recommence the work.

OFSI issued its first Russia-related penalty last month when it fined a concierge company £15,000 for having a sanctioned individual on its client list.

London-based firm Integral Concierge Services was found to have made or received 26 payments that involved a person whose assets have been frozen as part of the Russia sanctions.