Donald Trump Jr's meeting with a Russian lawyer while his father was running for election is the "proverbial smoking gun" needed to show he had committed a crime, a Harvard Law Professor has said.

Laurence Tribe said it suggested the President's eldest son "was part of [a] criminal conspiracy to violate [federal criminal] laws banning foreign aid to any US campaign."

Mr Trump revealed he had replied to an offer for incriminating material on Hillary Clinton from Russia by saying "I love it."

The younger Mr Trump released what he said was the full exchange leading up to his meeting with Natalia Veselnitskaya, a lawyer with links to the Kremlin.

He was told by a friend the information the lawyer had to offer would "incriminate Hillary and her dealings with Russia and would be very useful to your father."

The email added: “This is obviously very high level and sensitive information but is part of Russia and its government’s support for Mr Trump.”

Mr Trump replied: “If it’s what you say I love it especially later in the summer.”

Thousands march against Donald Trump in LGBT rights parade Show all 8 1 /8 Thousands march against Donald Trump in LGBT rights parade Thousands march against Donald Trump in LGBT rights parade A marcher hold a sign that reads "Love is Love is Love" on the National Mall with the Washington Monument in the background during the Equality March for Unity and Pride in Washington AP Thousands march against Donald Trump in LGBT rights parade Marchers unfurl a huge rainbow flag as they prepare to march in the Equality March for Unity and Pride in Washington AP Thousands march against Donald Trump in LGBT rights parade Marchers gather to hear speakers at the end of the Los Angeles LGBTQ #ResistMarch AP Thousands march against Donald Trump in LGBT rights parade Gil Mendez, of San Francisco, holds a sign to honor the victims of the shooting at the Pulse Nightclub in Orlando as he marches during the Equality March for Unity and Pride in Washington AP Thousands march against Donald Trump in LGBT rights parade An demonstrator looks on as LGBT members and their supporters take part of the Equality March for Unity AFP/Getty Images Thousands march against Donald Trump in LGBT rights parade A marcher listens to speakers at the end of the Los Angeles LGBTQ #ResistMarch AP Thousands march against Donald Trump in LGBT rights parade LGBT members and their supporters take part of the Equality March for Unity AFP/Getty Images Thousands march against Donald Trump in LGBT rights parade A marcher mugs for the camera during the Los Angeles LGBTQ #ResistMarch AP

US federal law makes it a "crime to receive anything of value... in connection with" an election, LawNewz reports.

Mr Trump and his lawyers have denied he did anything wrong.