Explosive texts messages have revealed the lengths US diplomats went to get Ukraine’s president to investigate the Democratic presidential hopeful Joe Biden this summer, weakening Donald Trump’s key rebuttal to the scandal.

The tranche of newly published messages shows that US State Department figures dangled the prospect of a White House meeting between Volodymyr Zelenskiy, Ukraine’s new leader, and Mr Trump while pressing for the probe.

From mid-July to early September the officials repeatedly pushed to secure an investigation into Mr Biden and his son Hunter, who once worked for a Ukrainian gas company, even going as far as to help draft an announcement to be put out by Ukraine.

However with almost $400 million of US military aid to Ukraine held back as talks continued, one senior diplomat begun to raise concerns, writing in one text: “I think it’s crazy to withhold security assistance for help with a political campaign.”

The messages from three US senior officials involved in Ukrainian policy and an aide to Mr Zelenskiy, which were released by congressional committees leading the impeachment inquiry, show the degree of arm-twisting happening behind the scenes.