Alistair Darling, Secretary of State for Social Security, is facing the threat of a rebellion over his plans, which would cut the numbers claiming widows' benefits over the next 25 years from about 270,000 to 30,000 - saving in total about pounds 500m.
"It could be as big as the row over lone-parent benefits, and this time we are ready," said one of the Labour MPs who is threatening to vote against the Government when the measure is brought before the Commons. "An awful lot of people didn't realise how big the cuts were going to be until a few hours later and they were really angry."
The cuts in widows' benefits, which will not apply to current claimants, are part of a massive welfare reform bill to impose more targeting of benefits on the needy, including the long-term sick, the disabled and widows, to be announced in Tuesday's Queen's Speech.
The Bill will pave the way for a drastic shift away from universal benefits to more means testing and targeting. It will take money away from better-off claimants in the middle classes and redistribute money to those near the poverty line.
There is growing alarm among Labour MPs that the cuts will hit those who are just above the income-support level. The MPs were infuriated by the way that Mr Darling presented the widows' benefits statement, effectively obscuring the scale of the long-term cuts by announcing an extension of the benefit to men for the first time.
The main losers will be women over 45 whose children have grown up and left full- time schooling. MPs were further annoyed to discover that men will only qualify for widowers' benefit in future if their wives had paid national insurance contributions.
The cuts also spell the end of the contributory principle in which people provided payments to a national scheme to draw on when they were in need. Many making payments now in national insurance contributions will not be able to draw on the benefits when they need them in the future.
MPs who attended the left-wing Campaign Group of Labour MPs this week after Mr Darling's statement reported that more MPs were joining the group. "There is mounting anger on the back bench" said one member of the group.
The left may be looking for an issue such as the welfare state on which to clip Tony Blair's wings after the controversy over the selection of candidates, and allegations of "control freakery" within the leadership.
Audrey Wise, who also played a leading role in the battle over cuts in one-parent benefits, was among the Labour MPs who expressed concern. But anxiety about the handling of the welfare cuts goes wider than the party's left-wing rump.