Ethnic minority voters have long been one of Labour's greatest electoral assets. In 2010, 68 per cent voted for the party, compared to just 16 per cent for the Conservatives. But in 2015 this pattern went into reverse. Labour's share declined to 52 per cent, while the Tories' more than doubled to 33 per cent (according to a British Future/Survation poll): the best result in their history.

This dramatic shift has attracted surprisingly little comment since the election but in a speech today Chuka Umunna will aim to change that. Addressing Unison’s 2016 National Black Members’ Conference, the former shadow business secretary will warn that Labour is "shedding votes from different ethnic minority communities to the Tories" and that it has "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."

Umunna, who will launch an independent inquiry into the issue with Keith Vaz (one of the four ethnic minority MPs first elected in 1987 and Britain's first Asian minister), will reveal new House of Commons library research showing that in 253 constituencies – more than one in three - the ethnic minority population exceeds the majority of the sitting MP.

On Labour's performance among BME voters in 2015, he will say: "Since 2005 the Conservative Party has been assiduously courting support across our different communities and it is yielding results. Conservative support amongst ethnic minority voters at the 2015 general Eeection jumped to 33 per cent - 1 million ethnic minority voters helped put David Cameron in Downing Street, the best result in that party’s history. Meanwhile our support dropped to 52 per cent. So an extraordinary jump for the Tories - a doubling of support - and a big drop in support for us. The alarm bells should be ringing."

The Tories have long argued that many ethnic minority voters are small-c conservatives open to voting for a Conservative Party free of the toxic associations of the past (Powell's "Rivers of Blood" and Tebbit's "cricket test"). Umunna will cite evidence from the Runnymede Trust showing that "more ethnic minority middle class voters agreed that a Conservative led government would lead to better economic policy." He will add: "In 2015 we extended our ethnic minority vote in heartland seats which already had large majorities but in marginal areas like Watford, Swindon and Milton Keynes - which we need to gain to win a majority - the Conservatives successfully extended their appeal to aspirational ethnic minority voters."

David Cameron made race equality one of the defining themes of his conference speech last year, denouncing the finding that "even if they have exactly the same qualifications, people with white-sounding names are nearly twice as likely to get call backs for jobs than people with ethnic-sounding names". The government has since pledged to introduce name-blind CVs for the civil service and for university applications.

In his speech, Cameron said of the Tories' childcare policy: "It was introduced by the black British son of a single parent, Sam Gyimah. He was backed up by the daughter of Gujarati immigrants who arrived in our country from East Africa with nothing except the clothes they stood up in, Priti Patel, and the first speaker was Sajid Javid, whose father came here from Pakistan to drive the buses."

Umunna will warn of the Tories' ambition to overtake Labour's ethnic minority representation. "In this Parliament, there are now 41 ethnic minority MPs: 23 Labour; 17 Tories. But, whilst there are 10 more ethnic minority Labour MPs, there are 15 more Tory ones compared to the last Parliament. Make no mistake: the Tories aim to ensure there are more Tory ethnic minority members of the 2020 Parliament than Labour ones."

Finally, he will defend Labour's record in office, arguing that "it 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.

"Now I can understand why, if you have never had need to use a children’s centre, or if every generation of your family has habitually gone on to university, if you have never been on the minimum wage or indeed your family has never suffered the racism of the police, why Labour’s achievements in office - and I could list many more - might not mean so much to you. But they made a fundamental difference to the lives of the people I represent.

"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. 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 per cent."