The extent of lobbying conducted by Shell and Rio Tinto in seeking legal support from the UK government to dismiss allegations of human rights abuses has been revealed in internal memorandums released by the Foreign Office (pdf).

The documents, obtained by the Corporate Responsibility Coalition (Core) and Amnesty International, show the closeness of the relationship between the multinational corporations and UK government departments. They also show that UK government officials were privately worried about being seen to be promoting business interests over ethical trading principles.

More than 60 pages of documents and letters from 2011 and 2012 have been obtained through freedom of information requests. The documents relate to two high-profile court cases against the firms at the US supreme court over alleged complicity in mistreatment of protesters and people who lived near extraction operations.

The lead case, Kiobel v Royal Dutch Petroleum Company, related to the fate of anti-Shell protesters in the Ogoni region of Nigeria who were executed by Nigeria's military junta in 1995. The second case was brought by environmental activists under the US's Alien Tort Statute against Rio Tinto, claiming that residents in Bougainville, an autonomous region of Papua New Guinea, suffered from pollution linked to the company's copper and gold mine.

The US Alien Tort Statute dates back to 1789 but had been used to pursue claims over violations of international law. In a landmark judgment last year, the US supreme court limited the law's reach by declaring that it cannot be used to sue foreign organisations for supposed wrongdoing on foreign soil. The cases against Rio Tinto and Shell were consequently dismissed.

In the runup to the hearings in Washington, Shell and Rio Tinto approached the Foreign Office and other ministries in London seeking UK government backing.

The FCO initially intervened at the US supreme court with a legal brief supporting Shell, pointing out the damage to British business of "extraterritorial jurisdiction".

The FCO documents say the submission "was not at the behest of Shell". The FCO then added a second, neutral brief – against Shell's wishes – making similar points of law.

One memorandum states: "While our argument in this case is a matter of sound legal principle, the perception of critics of HMG's position on business and human rights will be that we are standing up for big business and against the human rights cause of ordinary people."

Marilyn Croser, of Core, said: "While paying lip-service to corporate responsibility, the government was conniving with multinationals to reduce their exposure to litigation for the worst kind of abuses."

Peter Frankental, economic relations programme director at Amnesty International UK, said: "In these David versus Goliath encounters, the UK has effectively shielded the interests of powerful companies at the expense of their alleged victims."

Responding to the claims, the FCO said: "The UK intervened in this case to clarify our position on the proper limits of the extraterritorial application of US law. The UK initially submitted a brief in support of Shell because we were not advised that a neutral brief was a possibility. As soon as we became aware, we replaced our original brief with a neutral brief, as the best way of representing our wider legal concerns without taking a position on the specific allegations in this case."

A Shell spokesman said: "Shell companies have the right and the responsibility to make our position known to governments on any matters which affect us, our employees, our customers, our shareholders or local communities in a manner which is in accordance with our values and business principles. We have always denied, in the strongest possible terms, the allegations made by the plaintiffs in the Kiobel case."

Rio Tinto declined to comment on the documents.