The second most senior Republican in the US Senate has expressed disquiet about new bombshell testimony that Donald Trump made aid to Ukraine conditional on the launch of an investigation into his political rival, Joe Biden.

John Thune, who holds the position of Senate Republican whip, told reporters that the picture which was painted by America’s top Ukraine diplomat before the impeachment inquiry was “not a good one”.

The testimony in question, given by US diplomat William Taylor on Tuesday, had left Democrats declaring a “sea change” in their impeachment investigation and put Republicans on the back foot.

Mr Taylor, who was made 'charge d'affaires’ in the Ukrainian embassy in July, delivered an explosive opening statement that contradicted Mr Trump’s claim there was no “quid pro quo” between pausing almost $400 million in aid to Ukraine and demanding the country launch politically helpful probes.

Mr Taylor said that he had been told “everything” was dependent on the Ukrainians announcing the investigations which Mr Trump was seeking, including the release of the military aid.

One probe would have been into the company Burisma, which had employed Mr Biden's son Hunter Biden, and the other was into Ukraine's alleged meddling in the 2016 US election.