New source said to corroborate original informant's complaint about Trump's efforts to pressure Ukraine into investigating former vice president Joe Biden

House leaders warn US administration not to intimidate potential witnesses in impeachment probe, announce plans to subpoena White House over Ukraine scandal

In phone call central to impeachment inquiry, US president asked Volodymyr Zelensky 'to do us a favor,' said 'there's a lot of talk about Biden's son'

Senior intelligence official appears before congress to discuss 'urgent' concerns about president's communications with unnamed counterpart; Trump blasts report as 'fake news'

Party said to threaten to sue ex-members who spoke to BBC in upcoming investigation; Corbyn's deputy says 'silencing staff members' is 'as futile as it is stupid'