It extended its boundaries in 1866 and 1887 , and became a county borough under the Local Government Act 1888 .
Middlesbrough was incorporated as a municipal borough in 1853 .
Middlesbrough extended Middlesbrough 's boundaries in 1866 and 1887 , and became a county borough under the Local Government Act 1888 .
Middlesbrough was incorporated as a municipal borough in 1853 .