Nearly 20 percent of the Hub’s commercial real estate is vacant, and rents have dropped dramatically since the fourth quarter of 2008, according to new data.

For many tenants, it’s time to play let’s make a deal.

“It’s the economy, stupid,” said William F. McCall, president of McCall & Almy, a Boston brokerage that exclusively represents tenants. “We are not creating jobs. The main reason tenants move is because they are growing, and no one is growing in this economic environment.”

Average asking rents in the city’s central business district plunged to $44 per square foot in the fourth quarter, a 21 percent drop compared to $56 for the same period last year, according to CB Richard Ellis, a global commercial real estate firm.

At the same time, the amount of space available in the downtown swelled to 18 percent, up from 13.7 percent at the end of 2008.

The latest data reveals rents fell and vacancies increased in every major Boston sub-market, including the Financial District, Back Bay, Seaport, Charlestown, North Station and South Boston.

McCall said he encourages landlords to negotiate better terms with existing tenants or they might wind up with more empty space.

“I tell landlords they would be crazy not to lower the rent,” he said. “More space is coming vacant soon.”

Ropes & Gray will be vacating 400,000 square feet at International Place in 2011 as the large law firm moves to the Prudential Center, McCall said.

Joseph Fallon is still trying to fill his new 500,000-square-foot building on Fan Pier and Boston Properties is seeking tenants for its soon-to-be-completed building at Russia Wharf.

“Tenants have lots of choices,” McCall said.

If you have a business in Boston click the following to begin your search for great office space deals: Boston Office Space for Rent