Keep Sunday Special is planning a judicial review on the proposals, arguing the evidence behind them is out of date.
The government says changing the current law will give more powers to local councils and generate more money for the economy.
But John Ashcroft, who is part of the campaign, told Premier's News Hour he didn't think that was true.
He said: "Fundamentally people don't have money burning a hole in their pockets which they haven't got time to spend.
"The suggestion that there will be any increased turnover is unlikely."
The group has issued the government a letter before action claiming ministers did not carry out a genuine, unbiased consultation process.
It says the action has not been taken lightly and is done so with a heavy heart.
The Government has proposed that it devolves responsibility on the issue to local councils. That means they would have the power to relax the current laws and allow supermarkets to open for longer on a Sunday.
A number of church groups have raised their opposition to the plans.
Listen to John Ashcroft speaking to Premier's Alex Williams on the News Hour.