document: Baroness Morgan said Labour is in a serious situation if it cannot reconnect with working class communities.
  She said there "probably was an element" of Labour's Remain campaign starting too late.
  "A lot of us were exhausted", she told BBC Radio Wales.
  Senior Welsh Labour sources have privately said they were wary of talking about Brexit before the assembly election in case it drove Eurosceptic supporters towards UKIP.
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  Asked if Labour's Remain campaign started too late, she told the Eye on Wales programme: "I think there probably was an element of that.
  "And don't forget in Wales we had the assembly elections where people were not talking about the European Union. We were focused absolutely on those assembly elections."
  She added: "I think it was a mistake not to be talking about the European referendum in those assembly elections."
  Wales voted 52.5% to 47.5% for Brexit. Only five counting areas voted to Remain.
  The strongest Leave votes were in Torfaen (60%) and Blaenau Gwent (62%) - two traditional Labour strongholds in the south Wales valleys.
  The Welsh government has a programme to "re-energise" communities that voted to Leave, much of which would have been funded by EU money, Baroness Morgan said.
  "The question for us now is how on Earth do we reorganise the economy without that European funding?" she asked.
  "Unless we find a way to reconnect with that working class element within our communities then I think the Labour party is in a serious situation in future".
  Labour won 29 seats at May's assembly election - a better result than many expected - but lost 7.6% of its share of the vote on the constituency ballot.
  Earlier, on BBC Sunday Politics Wales, Baroness Morgan said Carwyn Jones was "absolutely, without question" the right man to lead Welsh Labour.