Labour has given the impression it takes support from ethnic minorities for granted and has not got a "hope in hell" of holding on to its current number of seats if it continues to lose votes from black and Asian voters, Chuka Umunna will warn.
The Conservatives have been "assiduously courting support" across different communities and it is "yielding results", the former frontbencher will say.
Some one million ethnic minority voters "helped put David Cameron in Downing Street" last May and Labour has no chance of winning in 2020 if the trend continues, Mr Umunna will tellUnison's National Black Members' conference.
In a thinly veiled attack on Jeremy Corbyn, he will also warn that while Labour may be able to "force the odd u-turn" on areas such as tax credits there is "no glory in opposition".
"In spite of all we have delivered, too often we have given the impression that we take our ethnic minority communities' support for granted. We cannot afford to do this," Mr Umunna will say.
"According to the House of Commons library, in 253 constituencies in the UK - more than one in three - the ethnic minority population exceeds the majority of the sitting MP. So, whether you can attract ethnic minority support in those seats can be decisive.
"Evidence from last year's general election suggests we are shedding votes from different ethnic minority communities to the Tories. And we have not a hope in hell of retaining all our current seats, let alone make any enough gains and winning the next general election if we continue to lose ethnic minority votes at this rate."
Tory support from ethnic minority voters at the general election jumped to 33% from 16%, the same percentage that Labour lost, Mr Umunna will say.
He will suggest there is evidence that middle class ethnic minorities believed a Conservative led government would lead to better economic policy and the party had won over "aspirational" voters.
The surge in support was the best result in the Conservative Party's history and the fall in Labour backing should have set "alarm bells ringing", he will tell the conference.
Mr Umunna will also defend the legacy of the Blair and Brown governments after deep divisions in the party have seen Mr Corbyn's supporters attack those on the centre as Tories.
The former leadership contender will add: "Labour - in and out of government - was not and never has been just another shade of Tory: the overwhelming majority of what we did delivered far greater social justice in Britain.
"That is our legacy and we should all be proud of it. Read all the equality impact assessments of our policies in government to see what I'm talking about."
"There is no glory in opposition - we can force the odd u-turn as we did on tax credits but the Tories are in the driving seat," he will add.
"That is why we must kick these Tories out in 2020, and - make no mistake - we will kick them out with a purpose: to fashion a politics of hope that brings together all communities around justice, peace and prosperity, for all Britons not just the top 1%."