The former private secretary to the Rwandan President has warned that British aid is bankrolling an unaccountable, repressive regime, accused of war crimes in neigbouring Congo and of killing or jailing dissidents at home and abroad.

David Himbara, who was President Paul Kagame’s right hand man until he fled the country two years ago, said: “Britain is not funding Rwanda. It is funding a dictator. It’s sustaining a bad regime by any measure. Let no British taxpayer flatter herself or himself that they’re helping Rwanda. No, you are merely extending their misery.”

Despite ever-more serious allegations of human rights abuse at home and war-mongering abroad, the UK is planning to give £270-million to Rwanda over the next three years. By then, aid to the country will have nearly doubled since the Conservative-led coalition came to power. Half of that aid will be given directly to the Rwandan government, to spend as it sees fit.

Rwanda relies on foreign aid for nearly half its budget and Britain is its biggest bilateral donor, providing about five per cent of the national budget – more than the Rwandan government allocates to defence spending.

“The United Kingdom’s aid to Rwanda is misplaced. It’s wrong. It cannot be justified,” said Mr Himbara, who now lives in South Africa and is on an alleged Rwandan government hit list. Speaking to Channel 4’s Dispatches in an exclusive interview to be broadcast on Monday Nov 26 he added: “How do you hold people accountable where there is no media, where there is no opposition party, where parliament is answerable to one man?”

The international development secretary, Justine Greening, is to decide next month on whether to hand over the next tranche of aid, totalling £21 million. Her predecessor, Andrew Mitchell, suspended payment of a £16 million instalment after evidence first surfaced that Rwanda was backing the violent rebellion in neighbouring Congo.

Controversially, Mr Mitchell then unfroze that aid in his last day in the job. In evidence to a parliamentary select committee he claimed to have had Downing Street’s approval.

Last week a United Nations report said the Rwandan military was supporting Congolese rebels who took over the eastern city of Goma.

Mr Kagame has long been feted for his leading role in ending the Rwandan genocide, during which an estimated 800,000 members of the Tutsi tribe were murdered by ethnic Hutus. and has been held up as a paragon of modernity and reform.

Both Mr Cameron and Mr Mitchell have travelled to the country to participate in Project Umubano, a Conservative Party social action programme, frequently meeting Mr Kagame. Mr Himbara described Mr Mitchell and other leading Conservatives as “Kagame’s groupies.”

But the relationship has been strained in recent months by accusations made by United Nations investigators over Rwanda’s alleged funding and arming of a rebel force in eastern Congo. This group, known as M23 and whose top commander is wanted for war crimes by the International Criminal Court, has forced at least 500,000 people to flee their homes and last Wednesday seized the regional capital, Goma.

The following day, the United Nations Security Council published its investigators’ findings, naming the Rwandan Defence Minister, General James Kabarabe – a close friend and associate of the President – as the alleged co-ordinator of rebel military operations inside Congo. Rwandan forces, the UN report claims, not only supplied guns, money and recruits, but has engaged directly in combat to help the rebels capture an expanding swath of territory.

The Rwandan government vehemently denies these claims, accusing the UN investigators of seeking to malign Rwanda’s image and dismissing the report as not worth the paper it was written on. President Kagame has joined regional leaders in urging the M23 to pull out of Goma, but has failed to condemn the rebellion.

“The report is not credible,” said Brigadier General Joseph Nzabamwita, Rwanda’s Defence Spokesperson. “It’s wrong. Based on a flawed methodology. The evidence is not there. It’s not there at all. The report is trash.”

In contrast, the British government says it judges the UN evidence to be “credible and compelling”.

On Thursday the William Hague, the Foreign Secretary, and Miss Greening said in a joint statement: “These allegations will necessarily be a key factor in future aid deisions to the Government of Rwanda.”

The promotion of regional peace and security as well as respect for human rights and democratic principles are built into the agreement which governs the aid relationship between Britain and Rwanda. Foreign policy officials claim British aid buys political leverage.

But, speaking in his first full interview since he fled Rwanda, Mr Himbara disagrees. “What leverage would that be?” he asks. “It has not stopped Kagame going into Congo. We have never seen UK say anything about Rwanda, no matter what it does. There is no evidence. [The] UK just gives blank cheques, it seems to me.”

Despite repeated requests, British aid officials in Rwanda were unable to provide specific examples of projects funded by British aid. Nor were they given permission to discuss them on the record. Requests for an interview with Miss Greening were declined.

Britain maintains that it considers the overall “direction of travel” when it looks at Rwanda’s performance on human rights. But there has been no public criticism of widely reported abuses, which have included the jailing of all three opposition party leaders, the decapitation of a deputy opposition leader, the shooting dead of a journalist, and the imprisonment of others. The repression intensified in the run-up to the presidential election in August 2010.

“There is no democracy in Rwanda. There is no political space in Rwanda. There is no free press in Rwanda. There is no independent judiciary in Rwanda,” says Ben Rawlence, an Africa expert at Human Rights Watch. “Senior opposition figures are all in jail, senior journalists are all in jail. There is no independent journalist operating any more. It’s a very scary place.”

David Himbara claims to have been the victim of attempted abduction by Rwandan agents and has been warned by intelligence officials in South Africa, where he now lives, that he is on a Rwandan government hit list. There have been at least seven assassinations or attempted assassinations of Rwandan dissident abroad since 2010. The former army Chief of Staff, narrowly survived assassination in South Africa last year.

Last year, London’s Metropolitan Police even issued “threat to life” warnings to two Rwandan exiles living in the UK. One, Rene Mugenzi – who fled the genocide in 1994 and who has had British citizenship for a decade – was warned: “Reliable intelligence states that the Rwandan Government poses an imminent threat to your life. The threat could come in any form.”

Mr Mugenzi, who stood as a Liberal Democrat candidate in a local election in London last year, had challenged President Kagame’s human rights record live on air in a BBC World Service radio phone-in. Although an alleged assassin was apparently intercepted and refused entry to the UK, he believes the threat is still active.

As a British taxpayer, Mr Mugenzi says: “I feel that I have sponsored my own assassination [and] also the oppression of my own people, because this money is propping up an oppressive regime.”

Asked whether, in light of a litany of serious allegations, Rwanda was complying with commitments it had made in its Memorandum of Understanding with Britain, Louise Mushikiwabo, Rwanda’s Foreign Affairs Minister, was resolute. “No question about it,” she said. “Rwanda has complied 100 per cent.”

She said her country should not be held to standards of behaviour that are unrealistic. “Rwanda is not an angel country – we’ve come from very far; a country where a million plus people get killed in a well-orchestrated genocide. We are moving at our pace with our own choices, not with anybody else’s.”

President Kagame appears to have taken a high stakes gamble. While the accusations against his country are serious, Rwanda will, from January, have a seat on the UN Security Council, which will boost its diplomatic clout and reduce the chances of it facing international isolation.

At a recent African economic conference in Kigali, the president told his audience: “I want to assure you [of] one thing. This new Rwanda: it does not respond well to blackmail.”