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 minister 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.”
Alexander Ehmann, of the Institute of Directors, said: “This is good news and a welcome step in the right direction.
“Companies facing problems have to be able to restructure swiftly, and a 45-day consultation period brings the UK closer to a number of EU competitors.
“We would have preferred a move to a 30-day consultation period – the same as for smaller-scale redundancies – which would have made the law less complex.
“The important thing now is that guidance is given making it clear when a consultation is to begin and end.
“Removing those employees on expired fixed-term contracts from this legislation is welcome, as their original inclusion was a particularly egregious example of the unnecessary gold-plating of European legislation.”
University and College Union general secretary Sally Hunt, said: “Casualisation in our universities and colleges remains the unacceptable underbelly of post-16 education and these changes send a very worrying message to staff.
“Employers should maintain collective consultations, whether or not they remain a legal requirement, as it is good for staff and institutions to engage in positive dialogue.
“The Government’s excuse that slashing the 90-day consultation period will make it easier for people to find another job is quite ridiculous considering the shocking state of the jobs market.”