After a surge of office leasing in the City of London last year, activity has declined, raising challenges to developments that have just opened or that are under way.

Tenants in the City leased more than 6.1 million square feet last year, the highest since 2006. But so far this year, there have been only 1.4 million square feet leased, a 51% decline from the three million square feet leased in the first two quarters of 2010, according to Jones Lang LaSalle.

“It’s good, steady leasing, but it’s downshifted,” says John Lutzius, an analyst at Green Street Advisors.

The decline …