A group of Bronx Terminal Market vendors plans to file a motion in court next week seeking to nullify the lease for the site between the city and a leading developer, Related Companies.

The vendors argue that the 63-month lease signed last year violated the City Charter because it bypassed the normal land-use process and was not open for bidding.

The action in state Supreme Court in Manhattan, if successful, would be a setback to Related’s plans to build and run a $394 million, 1 million square-foot shopping mall, in addition to a waterfront park and possibly a 350-room hotel, at the market’s current site. The company, one of the city’s most active developers, has begun the land-use application process and has said construction could begin as early as next year.

The city’s Economic Development Corporation estimated the shopping mall will produce $21 million in tax revenue annually for the city and create more than 2,100 permanent jobs in one of the city’s poorest neighborhoods. The Related project has the enthusiastic backing of the Bloomberg administration and the Bronx borough president, Adolfo Carrion.

A group of 18 of the 23 existing tenants has been dissatisfied with the $8 million relocation package that Related offered and wants to keep their shops together as one entity. According to the merchants’ attorney, Adrian Zuckerman of Lowenstein Sandler, among possible locations to house a new co-op market, the merchants are now evaluating a riverfront site, once envisioned as the site of a velodrome for cycling competition if New York City became host of the 2012 Olympics.

The site where the velodrome was to be built was transferred to the city in exchange for the site of the now-empty Bronx House of Detention, a few blocks from Yankee Stadium.

A spokesman for Related, Lee Silberstein, said no decision had been made on an alternative use of the velodrome site, and the city and the developer are still considering options.

Mr. Silberstein noted that several alternative market sites had been identified, including some in the immediate area, that could accommodate a group of the merchants, but the merchants continued to hold out.

“Some of the merchants have decided to take relocation, some have decided not to,” he said in a telephone interview. “They really have to think about and understand that there is not going to be a place where they will be able to locate where they can pay rents they are paying now.”

Mr. Zuckerman, the merchants’ attorney, criticized the land swap as a bad deal for the city’s taxpayers. The city’s tax assessment of the House of Detention land is $37.5 million, and the velodrome site is assessed at $11.6 million.

The Economic Development Corporation defended the exchange yesterday in a written statement: “Those numbers come from the Department of Finance and are for tax assessment purposes only. They do not take into consideration such things as remediation costs for environmental contamination or demolition costs that would be deducted if the property were sold for development.”

At a separate court hearing scheduled for August 30, the merchants plan to argue that they are paying too much rent in view of the market’s worsening conditions and the amount of rent Related pays the city for the site. Collectively, the current tenants pay $285,000 a month to Related, while Related pays the city $25,000 a month for rent. A lobbyist, Richard Lipsky, who is representing the tenants, said yesterday: “The tenants are actually funding the Related Companies to evict them from their market.”

The merchants’ rent has not been raised significantly since Related took over the property. A spokesman for the Economic Development Corporation, meanwhile, defended the level of Related’s rent, citing the $42 million the developer paid to buy out the previous lease, which had long been held by the Buntzman family.