It has a strong claim to be the world capital of everything from finance to design, but now London can add a new, more dubious distinction: it has become the reputation laundering destination of choice for foreign heads of state whose controversial activities may have stained their countries' public images.

An investigation by the Guardian has revealed that the capital's public relations firms are earning millions of pounds a year promoting foreign regimes with some of the world's worst human rights records, including Saudi Arabia, Rwanda, Kazakhstan and Sri Lanka.

They are earning as much as £2m per contract to provide communications advice to governments whose records on issues such as torture, corruption and free speech have been attacked by international organisations including the United Nations and the Commonwealth.

Politicians from Russia, Madagascar and China are among those to have sought out British PR firms to help burnish their image in what the Public Relations Consultants Association has identified as "a growing market" within Britain's £7bn a year PR industry.

Even Omar Bashir, the president of Sudan, wanted by the international criminal court on suspicion of crimes against humanity relating to the Darfur genocide, has approached two London firms, via representatives, asking for their help in managing his image.

"Autocratic governments are realising they need to be more sophisticated in the way they act rather than just telling people how it is," said Francis Ingham, chief executive of the PRCA. "There is great growth in the former communist bloc and in China."

One of the leading firms, Chime plc, headed by Lord Bell, Margaret Thatcher's former adviser, earned almost half of its £67m income last year from foreign contracts, up from 37% in 2008.