Hillary Clinton secured a boost from the first US presidential debate to lead Donald Trump nationally by three points, as advisers to the Republican confided they were frustrated by his performance and would prepare him more rigorously next time.

Mrs Clinton gained four percentage points in the wake of Monday night's showdown in New York which was watched by a record 84 million people on television.

According to a Politico/Morning Consult poll the Democratic nominee now leads by three points among probable voters.

However, only nine per cent of viewers changed their minds about who they were supporting because of the debate, with many waiting until after the final two debates to decide.