Evidence that the British government plotted to have a number of UK citizens sent from Afghanistan to Guantánamo Bay after the al-Qaida attacks of September 2001 has emerged in previously classified documents handed to lawyers representing victims of extraordinary rendition.

The documents show that Tony Blair's office intervened to thwart Foreign Office efforts to secure the release of a British Muslim detained in Zambia, with the result that he too was sent unlawfully to the American detention centre on Cuba.

While a number of senior figures in the last Labour government publicly criticised the use of Guantánamo and called for its closure, the documents show that in the immediate aftermath of 9/11 a decision had been taken at the highest level to use it to incarcerate British nationals.

The contents of some documents were made public at the high court today as lawyers for six rendition victims opposed an application by the government to halt their claim for damages temporarily. The government argues that proceedings should be delayed while attempts at mediation are made before a judicial inquiry into the UK's role in torture and rendition announced last week.

Tim Otty QC, counsel for five of the six men, said proceedings should be allowed to continue because the documents that the government is beginning to disclose shed new light upon the role that the UK authorities played in the men's mistreatment.

One document disclosed by the Foreign Office, dated 10 January 2002 and entitled Afghanistan UK Detainees, expressed what it described as the government's "preferred options". It states: "Transfer of United Kingdom nationals held to a United States base in Guantánamo is the best way to meet our counter-terrorism objectives, to ensure they are securely held." The "only alternative", the document adds, was to place these individuals in the custody of British forces in Afghanistan, or to return them to the UK. The court heard that a similar document was sent a week later to Sir Christopher Meyer, the British ambassador to Washington.

Otty said the contents of some of the documents "raise a number of troubling questions as to the role of the former prime minister's office in frustrating the release of one of the claimants", Martin Mubanga, who had been detained in Zambia early in 2002. "In the period of March and April 2002, the prime minister's office apparently countermanded a desire on the part of the foreign and commonwealth office to intervene on behalf on Mr Mubanga."

Number 10 had ordered that Mubanga, a dual British-Zambian national, should not be allowed to return to the UK, and the foreign office realised that if it sought consular access – thus acknowledging they regarded him as British – he would have been released to the UK authorities. One FO document complained about "the schizophrenic way in which policy on this whole case was handled in London", which had put the British high commission in Lusaka "in an impossible position".

By this date, said Otty, the risk of those falling into US hands being rendered and tortured was well known. "Despite that, someone at Number 10 saw fit to counter what the foreign office wished to do."

Mubanga was held in Guantánamo for 33 months. He and the other five men, including Binyam Mohamed, are suing MI5 and MI6, the foreign office, the home office and the attorney general over the role they played in their rendition and mistreatment.

The court heard that there was evidence apparently showing Blair's office or the UK intelligence agencies "playing some kind of role in determining the scope of consular access" to the men over subsequent years, as well as intervening in a manner that impeded their release.

Government lawyers have been contesting the six men's claims for more than a year. In May, after the appeal court dismissed attempts to suppress evidence of complicity in their mistreatment, the government indicated that it would attempt to settle out of court. Yesterday Rory Phillips QC, for the government and intelligence agencies, said compensation could be offered as a result of any mediation.