Nick Raynsford, the Minister for London, announced the boundaries for the Assembly seats as the Bill to set up the new body and a directly elected Mayor began its two-day committee stage in the Commons.
The move came after the London Labour Party agreed this week to a timetable for selecting candidates. The deadline for nominations is set for 28 February, with candidates to be chosen in September.
The selection procedure for Labour's mayoral candidates is due to be announced within the next few weeks and is likely to block Ken Livingstone.
Accepting the Local Government Commission proposals for the boundaries to be used in next May's elections, Mr Raynsford disclosed in a written reply that the constituencies would have electorates of up to 400,000.
They will each return an assembly member under the traditional first past the post voting system, with the other 11 members elected by proportional representation in a London-wide "top-up" constituency.
During the committee stage, looking at the first four clauses of the Bill, Mr Raynsford hailed the new assembly as the "smallest and most cost- effective of any city-wide administration in the world", adding that the 25-member assembly would be just one quarter of the size of the old Greater London Council.
But Eric Forth, the Tory MP for Bromley and Chislehurst, said the Government's plans would lead to a "Burden of bureaucracy which will damage the interests of Londoners".
The Government has proposed a number of amendments to the Bill, including a measure to guarantee funds raised from traffic charges and off-street parking will be spent on transport infrastructure for at least 10 years.