Half of US states have sued the Trump administration over its plans to halt funding for food aid used by more than 40 million low-income Americans.

The states hope to force the administration to use a roughly $6bn (£4.5bn) emergency contingency fund for the Supplemental Nutrition Assistance Program (Snap), also known as food stamps.

The US Department of Agriculture (Usda), which oversees Snap, has said it would not use it and allow funds to run dry in November, arguing the money may be needed for emergencies, such as natural disasters.

Republicans and Democrats have traded blame for the ongoing federal shutdown and there has not been any meaningful progress toward a deal.

“Bottom line, the well has run dry,” the Usda said in an announcement about Snap benefits on its website.

The suit, led by Democratic attorneys generals in 25 states and the District of Columbia, argues the administration not using the contingency funds would be unlawful and deprive millions of Americans from being able to afford groceries.

They note it would mark the first time in the programme’s history the funds would not be dolled out.

“Shutting off SNAP benefits will cause deterioration of public health and well-being,” the lawsuit reads.

“The loss of SNAP benefits leads to food insecurity, hunger, and malnutrition, which are associated with numerous negative health outcomes in children, such as poor concentration, decreased cognitive function, fatigue, depression, and behavioral problems.”

States involved in the lawsuit include: Arizona, California, Colorado, Connecticut, Delaware, Hawaii, Illinois, Kansas, Kentucky, Maine, Maryland, Massachusetts, Michigan, Minnesota, Nevada, New Jersey, New Mexico, New York, North Carolina, Oregon, Pennsylvania, Rhode Island, Vermont, Washington and Wisconsin.

“While Donald Trump parades around the world trying to repair the economic damage he’s done with his incompetence, he’s denying food to millions of Americans who will go hungry next month,” California Gov Gavin Newsom said.

“It’s cruel and speaks to his basic lack of humanity. He doesn’t care about the people of this country, only himself.”

Responding to the lawsuit, the Usda blamed Democrats for the fund running dry and said they need to decide whether they want to “hold out for the Far-Left wing of the party or reopen the government so mothers, babies, and the most vulnerable among us can receive timely WIC and SNAP allotments”.

Even if the contingency fund is used to help Snap beneficiaries, it would only cover about 60% of one month of benefits, according to the Center on Budget and Policy Priorities (CBPP), a think-tank focused on policies that help low-income families.

Snap works by giving people reloadable debit cards that they can use to buy essential grocery items.

A family of four on average receives $715 (£540) per month, according to CBPP, which breaks down to a little less than $6 (£4.50) per day per person.

The states administer the programmes, with much of the funding coming from the federal government.

Several states have pledged to use their own funds to cover any shortfall, however the federal government has warned that they will not be reimbursed.

Some, including Massachusetts – where one million people are expected to lose benefits – have said they do not have enough money to make up for the lack of funds.

Many states are working with people in Snap to try locate charity food pantries and find alternative sources for meals, and California is deploying its National Guard to help distribute food.

The US government shutdown entered its 28th day on Tuesday, making it the second-longest shutdown in history.