Britain must answer in court claims that it is turning a blind eye to UK firms that trade in lucrative Congolese "conflict minerals", a campaign group announced today.

The group, Global Witness, said the firms continued to buy minerals from rebel militias, prolonging a 12-year conflict. It planned to sue the government for failing to refer the firms for UN sanctions.

The move comes amid global efforts to halt the trade in rebel-controlled minerals such as tin, gold, and coltan, in the Democratic Republic of Congo, where millions have died since civil war erupted in 1998.

"British companies have supported armed groups by purchasing minerals from areas under their control in the DRC," Global Witness says in an application to the high court for a judicial review. "Despite this, the UK government has never put any of them forward for sanctions."

It claimed that the British government was "acting unlawfully", refusing to put forward individuals as well as firms. Some UK companies, known to have been trading in minerals sourced from the eastern DR Congo, should have been referred to the UN sanctions committee in 2008 and 2009, the campaigners said.

Gavin Hayman, campaigns director at Global Witness, said: "It is a sad day when we have to sue the UK government, but we hope this case will mark a turning point. The issues at stake have global significance for how wars are financed. These companies have profited from a brutal conflict, and should face UN sanctions – but sanctions are useless without a fair and clear government procedure for considering whether individuals or entities should be listed."

The UN sanctions would impose travel bans and asset freezes on those supporting illegal armed groups in DR Congo. The US has adopted laws requiring American firms involved in DR Congo to certify that they do not buy conflict minerals. A UN panel is also developing guidelines to ensure minerals can be traced.

A report by Global Witness last year called for UN sanctions against foreign companies buying the minerals from intermediaries without checking provenance. Hayman said : "The link between natural resources and conflict in the Congo is well known. Armed groups controlling the trade in minerals like tin and tungsten use the money to buy guns and fund their campaign against civilians.

"The UN resolutions recognised that companies sourcing directly or indirectly from the region are part of the problem. But in spite of our frequent appeals, the UK government has steadfastly refused to act, which left us no choice but to take them to court."

The British government, one of DR Congo's biggest donors, said it would treat the allegations against companies on a case-by-case basis.

A Foreign Office spokesperson said: "The government expects all British companies in the minerals sector in the DRC to follow high standards of due diligence, and to make every effort to establish the route through which the minerals they buy have passed."

"We will continue to take reports that they are not doing so seriously, and will assess in each case whether there are grounds to consider recommending to UN partners that sanctions measures be imposed, or supporting proposals for listings made by other states.

"Bringing DRC's natural resources more fully under state control is a key theme of our work in the country. We support a number of projects to establish better management of the country's minerals, and are looking to deepen our involvement in this area."