The council plans to end grants and service-level agreements to members of Birmingham Advice Network at the end of next month and establish a new commissioning process in July at the earliest.
Nicole Catenazzi, Compact and public law advocate at the NCVO, wrote to the council this week expressing "serious concerns about the decision-making process and the legality of leaving a substantial gap in service".
Her letter says: "The effect of having no funding from the council for a period of five months or more will be that most of the affected organisations will have to close permanently.
"It appears that there was no consultation carried out in advance of this decision, nor were any impact assessments performed."
Jack Dromey, the Labour MP for Birmingham Erdington, told a Westminster Hall debate yesterday: "It would appear that the cuts are not just cruel and callous; they may be unlawful as well.
"Many of us will give evidence in any proceedings that are held because we know from our experience that there was no proper consultation in advance, and that no serious impact assessments were conducted.
"The council made the decisions just before Christmas, and it has gone hell for leather to implement the next stages without proper consultation or impact assessments."
A statement by the 13 organisations said. "The council has not revealed the size of the cut to the overall budget, but there are indications that it may be as much as 50 per cent."
Anne Limbert, head of operations at Birmingham Citizens Advice Bureau, which faces a £600,000 cut, said it had planned to close four of five bureaux in the city today, with the loss of 45 jobs, but this had been put on hold pending the outcome of emergency talks with the council next Monday.
"There will be huge issues around support for the most vulnerable in our society, but there has been no dialogue at all with the city council on that," said Khalid Mahmood, the Labour MP for Birmingham Perry Barr at the debate.
Business minister Edward Davey would not respond to questions on whether the cuts were compliant with the Compact, the public and voluntary sector fair play agreement.
A spokesman for Birmingham City Council said: "We've always made it clear to agencies that funding was not guaranteed beyond any single year, up to a maximum of three years. It was never our intention for agencies to become dependent on this source of funding or that it form their sole source of income.
"The council's contracts with the current providers ceased in March 2010 and were rolled over until 31 December. From then, a funded notice period of 90 days was in place to provide some protection for the providers, and CAB was paid £150,000 notice payment to enable it to continue to offer services in the interim period.
"Individual meetings are being held with the service providers, including the CAB, to engage them with the recommissioning process."
The 13 organisations affected are: Afro-Caribbean Millennium Centre; Age Concern Birmingham; Age Concern Perry Barr; Azad Kashmir Welfare Association; Bangladesh Welfare Association; Birmingham Asian Resource Centre; Birmingham CAB; Birmingham Settlement; Birmingham Tribunal Unit; Birmingham TUC Centre for the Unemployed; Chinese Community Centre - Birmingham; St James Community Support & Advice Centre; Shaheed Udham Singh Welfare Trust.