A Ukrainian ex-prosecutor general has told the BBC there is no reason for his country to investigate President Donald Trump's political rival Joe Biden. Yuriy Lutsenko said any investigation into Mr Biden and his son would have to start in the US. "I don't know any reason to investigate Joe Biden or Hunter Biden according to Ukrainian law," he said. Mr Trump's efforts to have Ukraine investigate the pair prompted an impeachment inquiry by the Democrats. A transcript of a call Mr Trump made to Ukraine's new President Volodymyr Zelensky on 25 July shows he urged him to investigate discredited corruption allegations against Mr Biden and his son. Mr Trump and his allies have been suggesting that Mr Biden, as Barack Obama's vice-president, encouraged the firing of Ukraine's top prosecutor, Viktor Shokin, because he had been investigating Ukrainian natural gas company Burisma, which employed Hunter Biden. Hunter Biden became a director at Burisma in 2014, while his father held a key role in US policy towards Ukraine. Mr Biden is currently frontrunner to be the Democrat to take on Mr Trump in the November 2020 presidential election.