City MP Jack Dromey has urged the Chancellor to stop tax credit cuts which it is suggested will leave a million people poorer as London's Tory mayor Boris Johnson joined in the criticism.
The high-profile mayor is the latest Conservative to express concern about the cuts which take money away from an estimated 107,000 working people on low salaries.
Tories fear there will be a backlash from voters when they receive letters in December telling them how much they will lose.
Birmingham MP Jack Dromey (Lab Erdington) said: "As the grim reality of millions losing their family income dawns, more and more Tory voices are speaking out.
"But what matters most is whether they will join Labour in opposing this cruel measure which will hit children particularly hard."
Around 107,000 West Midlands families could lose £3,000 a year under cuts to tax credits, according to trade union Unison.
Chancellor George Osborne, who introduced the change, has also announced a new living wage for working people aged 25 and over.
This is £7.20 an hour and rises to £9 an hour by 2020, compared to the existing minimum wage of £6.50.
But experts say many families will still be worse off even after all the changes are taken into account.
Think tank the Resolution Foundation says more than a million households will lose an average of £1,350 a year.
Conservatives including David Davis, who once stood for the party leadership, have criticised the change.
Mr Johnson will set out concerns in his speech to the Conservative conference in Manchester.
He will say: "We must ensure that, as we reform welfare and we cut taxes, that we protect the hardest working and lowest paid.
"Shop workers, cleaners, the people who get up in the small hours or work through the night because they have dreams for what their families can achieve - the people without whom the London economy would simply collapse.
"(These are) the people Labour is leaving behind and then there is an even more important requirement.
"If people are to feel bound in to this system then there must be hope and aspiration and, above all, there must be opportunity and it is here that we Tories have a massive advantage.
"Because, if Labour is once again becoming the party that pointlessly bashes the rich, it is we who give everyone the tools to make their own lives and their own successes."
The London mayor is expected to challenge Mr Osborne for the Conservative Party leadership once Mr Cameron stands down before 2020.
Senior Tory says cuts will make British people work as hard as Chinese
There was anger after Health Secretary Jeremy Hunt defended the tax credit cuts by suggesting they would make British people work as hard as the Chinese.
He later said his words had been misinterpreted by the media.
Speaking at a fringe event in Manchester, Mr Hunt said: "We have to proceed with these tax credit changes because they are a very important cultural signal.
"My wife is Chinese. We want this to be one of the most successful countries in the world in 20, 30, 40 years' time.
"There's a pretty difficult question that we have to answer, which is essentially: are we going to be a country which is prepared to work hard in the way that Asian economies are prepared to work hard, in the way that Americans are prepared to work hard?
"And that is about creating a culture where work is at the heart of our success."
He added: "It matters if you are earning that yourself because if you are earning it yourself you are independent and that is the first step towards self-respect.
"If that £16,500 is either a high proportion or entirely through the benefit system you are trapped. It is about pathways to work, pathways to independence."