Earlier this month, a letter from Conservative MP Julian Knight, supported by 70 mainly Tory MPs, accused the BBC of a biased and "pessimistic" view.
But the counter letter - signed by 85 MPs and MEPs - called on the corporation to defend its independence.
They urged the BBC "not to succumb to pressure to skew" coverage.
The letter to director general Tony Hall warned of "an authoritarian attempt to force people to accept a single version of events".
Labour MP Pat McFadden, who coordinated the response, said: "This letter is a strong statement of support for the BBC to be free to do its job to report fearlessly and impartially.
"The reason the BBC is respected as a news source around the world is precisely because it is independent.
"And at a time when those who ask fair and tough questions are being denounced, that independence and impartiality is more important than ever."
A BBC spokesman said: "As we say to politicians from all sides, impartiality is the cornerstone of BBC News and we go to great lengths to ensure that we balance our coverage and address all issues from a wide range of different perspectives."
Among the MPs who signed the letter were Conservatives Kenneth Clarke, Nicky Morgan, and Anna Soubry; Labour's Yvette Cooper and Ben Bradshaw; the Liberal Democrat leader Tim Farron; and the SDLP's Mark Durkan.
A cross-party group of MPs has defended the BBC, after another group accused it of biased Brexit coverage.