Labour must reach beyond its own activists if it wants to win elections, Sadiq Khan has warned.
The new London Mayor said internal party squabbles "mean little or nothing to the huge majority of voters".
Writing in the Observer newspaper, Mr Khan also criticised the electoral campaigns of Zac Goldsmith and David Cameron.
He said they "used fear and innuendo to try to turn different ethnic and religious groups against each other".
Mr Khan compared these tactics to something "straight out of the Donald Trump playbook".
Mr Khan, the capital's first Muslim mayor, also said Labour only won when it faced outwards and that, however tempting, the party must resist focusing on internal issues.
Meanwhile, Labour leader Jeremy Corbyn is set to meet with Mr Khan on Sunday, having missed his inauguration ceremony at Southwark Cathedral the day before.