Employment Relations Minister Jo Swinson said the move, along with other changes, was aimed at helping workers and businesses.
But the TUC said the last thing the country needed was for the Government to make it easier to sack people.
Plans were also announced to exclude fixed-term contracts from collective redundancy agreements when they reach the end of their "natural life".
The LibDem minister, MP for East Dunbartonshire, said a consultation on the changes had produced a strong argument for shortening the 90-day period, adding: "The process is usually completed well within the existing 90-day minimum period, which can cause unnecessary delays for restructuring, and make it difficult for those affected to get new jobs quickly.
"Our reforms will strike an appropriate balance between making sure employees are engaged in decisions about their future and allowing employers greater certainty and flexibility to take necessary steps to restructure."
The replacement of the current 90-day period to 45 days will still allow full employee engagement and offer employee representatives a statutory right to contribute to the process, said the Business Department.
The change is planned to be introduced in Britain from next April.
TUC General Secretary Brendan Barber said: "The last thing we need is for the Government to make it easier to sack people. Unemployment has not gone as high as many feared because employers have worked with unions to save jobs, even if it has meant sharing round fewer hours and less work.
"The need to consult unions has made an important contribution to that, and also given staff, many of whom will have had years of loyal service, time to think through their options.
"These measures will not create a single extra job. The idea that an employer will change their mind about taking someone on because the statutory redundancy consultation period has been reduced from 90 to 45 days is close to absurd.
"Removing consultation rights from fixed-term contract staff will seriously increase job and financial insecurity for vulnerable groups of workers, and temporary staff will lose out on redeployment opportunities."
Tim Thomas, head of employment and skills at EEF, the manufacturers' organisation, said: "Today's announcement will send a strong signal to industry that the Government is committed to creating the flexible labour market that it needs. By reducing the consultation period from 90 to 45 days, the Government has taken a further step to creating a modern, consultation system based on the quality, not the length of, the process.
"It now needs to complete the package by introducing simpler settlement agreements and protected conversations that provide a better base for discussing and managing change in the workplace."
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