Sir Philip Hampton reportedly told US politicians that RBS directors had failed in their “fiduciary responsibilities”, and had not carried out proper due diligence before the ill-fated purchase of Dutch bank ABN Amro.

Earlier this month, the Financial Services Authority found no “failure of governance on the part of the board at RBS”, which was one of the highest-profile casualties of the financial crisis.

However, in the diplomatic cable’s account of a meeting between Sir Philip and three US officials, he appears to acknowledge several serious failings in the bank’s conduct.

“Top among them was its heavy exposure in the US subprime market and the bank’s purchase of ABN Amro, which occurred at the height of the market and without RBS doing proper due diligence prior to the purchase,” the leaked document states. “The board of directors never questioned this purchase, which Hampton termed a failure of their fiduciary responsibilities.”

Sir Philip, who was appointed chairman of RBS in 2009, was said to have made the comments on September 11, 2009, nine months into his new role, during a meeting with Democrats Paul Kanjorski and Luis Gutierrez and Republican Scott Garrett.

Also uncovered on the WikiLeaks site was the claim that British police helped “develop evidence” against Madeleine McCann’s parents while they were being investigated by the Portuguese as suspects in the case of their daughter’s disappearance.

The US ambassador to Portugal was apparently told by his British counterpart, Alexander Wykeham Ellis, that UK officers had co-operated closely with their Portuguese colleagues.

The US official, Al Hoffman, reported: “Without delving into the details of the case, Ellis admitted that the British police had developed the current evidence against the McCann parents, and he stressed that authorities from both countries were working co-operatively.”

Many people at the time were under the impression that Portuguese police were behind the drive to name the McCanns as arguidos, or formal suspects in the country’s legal system.

The couple have said that there was “absolutely no evidence to implicate them in Madeleine’s disappearance whatsoever”.

A spokesman for Kate and Gerry McCann said last night: “To this day, they continue to work tirelessly on the search for their daughter, co-operating when appropriate with both the Portuguese and British authorities.” Their arguido status was lifted in July 2008, and Portuguese authorities ended their investigation into the couple.

WikiLeaks also claimed to reveal US concerns that the UK was struggling to cope with homegrown extremism, with Americans warning that Britain had made “little progress” in engaging with the UK’s Muslim community in the year after the July 7 bomb attacks in London.