Senior government figures in Ukraine, whose government has received billions of dollars of international funding since the uprising that toppled former dictator Viktor Yanukovych, have been accused of massive political and financial corruption by a senior former ally.

Pro-Western president Petro Poroshenko’s office strenuously denies the allegations, which have been made by former parliamentarian and oligarch Alexander Onishchenko. Mr Onischenko has fled to London where he now lives in exile having himself been accused of corruption by Ukrainian authorities.

In a series of extraordinary allegations, Mr Onishchenko claims his candidacy to become a Ukrainian MP was suddenly able to overcome administrative hurdles after he paid a senior parliamentarian $6m.

He also claims that he was asked by the senior government figure to orchestrate and fund a smear campaign against the former Prime Minister Arseniy Yatsenyuk. The campaign, he said, cost nearly $3m a month and ran for nearly 10 months. Mr Yatsenyuk resigned in April this year after his popularity collapsed.

The President’s office said that Mr Onishchenko was attempting to politicise the criminal case against him and “pretends to be a victim of political repressions. This is the motivation for his absolutely false accusations.”

The Independent has been unable to verify Mr Onishchenko’s claims and is not naming the senior figure or his intermediaries for legal reasons.

His allegations come as concerns have been raised by the European Union, International Monetary Fund and others in the international community that not enough is being done to tackle endemic corruption in the country.

Mr Poroshenko’s administration has been criticised for failing to prosecute a single member of Mr Yanukovych’s regime and Transparency International ranked Ukraine 130th out of 168 countries in its 2015 corruption index.

Mr Poroshenko’s administration has received some praise however for anti-corruption measures it has adopted, including the establishment of the National Anti-Corruption Bureau (NABU) and public procurement reform. But these organisations themselves have come under criticism for failing to curb endemic white-collar crime in the country, which has been putting off Western investors.

NABU claims Mr Onishchenko organised the embezzlement of state funds in a joint programme with the gas company UkrGasVydobuvannia. It claims the scheme led to losses of around £50m to the state. NABU detectives also seized 10 apartments, nine houses, 10 pieces of land, 39 bank accounts, and 11 cars belonging to him and his suspected accomplices.

Mr Onishchenko claims the case is fabricated and politically motivated.

The 47-year-old father of two was born in a village near Rostov in Russia – horse-breeding country. Although he lived most of his life in Ukraine, the native love of horses stayed with him, and he later became a champion showjumper and breeder. He claims his candidacy to become an MP was in jeopardy in 2014 because he had been out of the country representing Ukraine in the London 2012 Olympics equestrian team. His absence, he claimed, meant he had fallen foul of electoral rules about how long he had resided in Ukraine.

But two days before the deadline for prospective parliamentary candidates was up, Mr Onishchenko says he got a call from a senior government figure: ‘You should come, you should meet my guy, he will explain what to do.’”

He claims: “I meet this guy and he told me I must pay this amount to support the President in the election – $6m.”

Asked how he can prove it, Mr Onishchenko says he has nothing written down but he would be willing to take a lie-detector test.

Asked how he felt about making such a payment, he responds: “We had this habit for 20 years. It was always the same system. They [the government] have the power, they have the tax administration, they have the general prosecutor. If they want to make problems for you, they will. They can destroy you completely. You have to find some way to get good relations.”

He said that under the Yanukovych regime, businessmen like him had to give half their post-tax profits to the president.

He describes the current mechanism of bribery as if it were commonplace. Asked how the $6m alleged bribe was paid, he chuckles: “Actually, that time we paid in Hvrina [the local currency], so it was like five or six bags.” He spreads his hands the width of a sports bag: “You get special bags from banks to carry a lot of money.” The money was paid in cash, meaning there was no paper trail, he said.

Mr Onishchenko claims other alleged kickbacks were slightly more subtle. He alleges the senior government figure demanded payments through intermediaries of 2,000 hvrinas (around £60) per 1,000 cubic meters of gas he sold, showing The Independent a series of SMS messages he claimed were instructions from an intermediary about the payment.

In addition to that, he claims, an agent of the senior government figure would personally come to his office and carry away $1m in cash for “pocket expenses”.

He says he demanded to know what he would get in return for his $6m, pointing out that previous payments had not delivered a return. The government figure said: ‘No, no, no don’t worry, we’ll talk about gas business, you can get priority.”

When asked why he wanted to become an MP, Mr Onishchenko answers with alarming honesty: “To protect my business.”

He says it all went well at first, but then, he alleges, the senior government figure said there was a blockage to their business plans: the Prime Minister, Arseniy Yatsenyuk.

Mr Yatsenyuk, an economist, lawyer and long-term parliamentarian, was the first Prime Minister after the Maidan revolution.

But according to Mr Onishschenko, the senior government figure was convinced the PM had to go. He claims the official told him: “‘He is our enemy and will destroy everything we want to do… you should help me to kick him out, because with this guy we’re never going to get anywhere.’”

So, Mr Onishchenko says, he organised a propaganda department against the elected Prime Minister. “I killed him in one year,” he says, claiming Mr Yatsenyuk’s ratings fell from 30 per cent to 1 per cent.

With his $3m-a-month budget, he says he funded anti-Yatsenyuk protest rallies, paid for ultra-right nationalists to carry out anti-government actions and bribed media outlets to run anti-Yatsenyuk stories.

Explaining how demonstrations can supposedly be created in Ukraine, he says: “You have to pay people to come, and you have to pay the TV to cover it. The same as you have to buy advertising or something. TV and newspapers.”

Asked how much people were paid to attend, he is slightly vague: “About $50 or something. There are companies who organise this.”

According to Mr Onishchenko, his propaganda machine dredged up practically every enemy Mr Yatsenyuk had ever made and put them in the media to attack him. Were the allegations true? “I don’t know exactly what was true or not. I never checked. For me, the reason was just to make a scandal.”

Apparently it was not just old enemies that he roped into the campaign. He claims his group would also help arrange mass demonstrations to highlight political errors Mr Yatsenyuk might make.

“For example,” he says, “he put in parliament some laws that meant people paid the highest rates for water and energy, so we organised big rallies against him… We saw his mistake and we used it. You see, when you make a mistake sometimes nobody sees it. But if I want to show you up I will make a big problem from your mistake. I put it everywhere. I create demonstrations showing “Big Mistake! Big Mistake!”

Mr Yatsenyuk was replaced in April by Volodymyr Groysman, a candidate nominated by the President, albeit only after his first choice was rejected by the governing coalition.

The relationship between the senior government figure and Mr Onishckenko soured earlier this year for numerous reasons, Mr Onishchenko says. One was apparently that he had failed to persuade his ally Yulia Timoshenko to adopt certain policies.

“He asked me to be close to Timoshenko, to protect him from her, because I had a good relationship with her. But he always asked me to do something which she didn’t want to do… I couldn’t get any control over her and she didn’t want to talk to him.”

Ms Timoshenko has publicly commented on Mr Onishchenko’s case, stating: “These days businesses are raided just like under Yanukovych, assets are grabbed. Today they seize Onishchenko’s assets. There is a lot of buzz about Onishchenko, but today there are many entrepreneurs whose businesses are being raided.”

Mr Onishchenko said he feared he would be arrested so he fled the country on 16 June. He and his mother Inessa – who he says is also living in exile in London – have both been served with notices for their arrest by the Ukrainian authorities. Several alleged co-conspirators have been arrested.

The presidential office described his allegations as “expedient fictions”. It also claimed Mr Onishchenko had recently acquired Russian citizenship and was “a tool of Russia’s hybrid war against Ukraine” – both of which claims Mr Onishchenko strenuously denies.

Mr Onishchenko, who has not sought asylum, said he hopes Ukraine will attempt to extradite him from the UK so that he can make his allegations fully in the English courts.

His claims come soon after the charismatic governor of Ukraine’s Odessa region, former Georgian president Mikhail Saakashvili resigned, accusing Mr Poroshenko of backing corrupt officials.