LONDON (Reuters) - Leading Brexit campaigner Nigel Farage said on Tuesday he was in a good position to help Britain build ties with Donald Trump after the U.S. President-elect tweeted that Farage should be British ambassador to Washington.

"I have known several of the Trump team for years and I am in a good position with the President-elect’s support to help," Farage, leader of the opposition UK Independence Party (UKIP), wrote in a column for the Breitbart website.

"The world has changed and it's time that Downing Street did too," he said, referring to Prime Minister Theresa May's office.

Downing Street earlier said there was "no vacancy" for an ambassador to the United States and an excellent one was already in place.

Trump, who after his election victory met Farage ahead of any EU leaders, had said on Twitter that "many people" would like to see the UKIP leader, who is on very bad terms with the ruling Conservatives, as Britain's ambassador.

(Reporting by Michael Holden, editing by Estelle Shirbon)