Sayeeda Warsi, a former foreign office minister who has played a key role in the Conservatives' appeal to Muslim voters, last week embarrassed Cameron by resigning via Twitter over what she said was the government's "morally indefensible" approach to the conflict between Israel and Hamas.
On Sunday she dominated the front pages of Britain's newspapers again, this time warning that what she called Cameron's neglect of ethnic minority voters would cost him an outright victory at next year's national election.
"The electoral reality is that we will not win outright Conservative majorities until we start attracting more of the ethnic vote," she said in an interview with the Sunday Times and Independent on Sunday newspapers.
"We've probably left it a little too late to take this part of the electorate seriously."
The criticism from Warsi, the daughter of Pakistani immigrants to Britain who speaks fluent Urdu, Punjabi and Gujarati, will hit a sore spot for Cameron.
Poor support from ethnic minority voters - which make up around 14% of the electorate - was one factor that cost Cameron an outright win in 2010, forcing him into a coalition government with the left-leaning Liberal Democrat party.
An academic study showed that only 16% of ethnic minority voters backed the Conservatives in 2010, while Labour, the main opposition party, won 68% support.
Current opinion polls give Labour a lead of around 4 percentage points over the Conservatives going into the May 2015 vote.
Cameron's political spokesman would not comment on the interview, but pointed to a letter last week in which the prime minister said he was sorry that Warsi had decided to resign and thanked her for her work in government.
Warsi, a baroness who sits in the upper house of parliament, in 2010 became Britain's first Muslim to serve in cabinet but was later demoted to be a senior minister of state at the Foreign Office and a minister for faith and communities. She also chaired the Conservative Party between 2010 and 2012.
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