MP for Hemsworth Jon Trickett revealed the exchange during a fringe event at the Labour Party conference in Brighton yesterday.
Mr Trickett, speaking about regaining Labour heartland votes, said: “I will not accept that [... northern ‘backwards’ people need to be protected.
“A very senior member of the Labour Party, she said to me: ‘Well no wonder they’re all coming down south, the young people, because you can’t be gay up north’.
“That was said by somebody whose name you will have mentioned several times in the last few weeks.
“And she then said: ‘All your voters are all racists’.”
Mr Trickett’s constituency voted 61 per cent leave in the European Union referendum but he said many of these votes were down to a changing economy, where workers no longer knew their place.
He said: “Those people who suggested that the people who voted for Brexit did not know what they were voting for, it sort of infantalises 17 million people.”
It comes as rifts within the Labour movement over Brexit have deepened ahead of a showdown over whether the party should campaign to stay in the European Union.
Labour leader Jeremy Corbyn has called for a neutral position going into a general election, saying that he would negotiate a new Brexit deal with Brussels which would then be put to a referendum.
Under his approach the party's position on how to campaign would not be decided until a special conference, after the expected general election.
But delegates at Labour's conference in Brighton will vote on whether the party should decide now on whether to campaign to stay in the European Union, even if that means rejecting a deal Mr Corbyn has negotiated with the EU.
Shadow chancellor John McDonnell, who has said he will campaign for a Remain vote in the promised referendum, said the process put forward by Mr Corbyn was "logical" and insisted "there isn't any war in the Labour Party" over the issue.
Shadow cabinet ministers including Emily Thornberry and Tom Watson have called for the party to back a Remain vote now, rather than wait for a special conference after the election.
Their actions led to Unite union boss Len McCluskey suggesting they should either get in line or "step aside" from their shadow cabinet roles.
It is understood that Unison will vote against the NEC statement on Brexit later today, and support the motion calling for Labour to campaign for remain.
A source said the move was aimed at giving a clear and unambiguous message on Brexit to help bring about the election of a Labour government.
Most other unions are expected to support the NEC statement.
Mr Corbyn's NEC statement was emailed round the body and endorsed without a formal meeting on Saturday, despite opposition from some members.
Jon Lansman, boss of the Corbyn-supporting Momentum campaign group and an NEC member, said the process had been a "travesty".