The Dungavel Immigration Centre is to be expanded to house more asylum seekers and their families.
Dungavel's capacity will increase by 25% through its new 44-bed unit
The plans for the Lanarkshire centre were revealed by Immigration Minister Beverley Hughes in a written answer to the Conservatives.
The proposals will increase Dungavel's capacity by 25% with the construction of a new 44-bed unit on the site.
The new block will cost £3m and consist of pre-fabricated buildings with bars on the windows.
The Home Office previously admitted it was considering an expansion of the centre's facilities but insisted it was just one of a number of options under consideration.
But Ms Hughes' letter, released on the final day of the parliamentary session last week, confirmed the plans are to go ahead and they are close to completion.
Her reply to Shadow Scottish Secretary Peter Duncan MP stated that planning permission for the extension had been secured and technical changes to the construction contract were being discussed.
She wrote: "We have planning permission to install a 44-bed ready-to-use unit on the Dungavel site.
"Variations to the contract will be required to take account of the increase in population.
"Negotiations for the contract variation are ongoing."
Bad news 'buried'
In response Mr Duncan said: "It's typical of this Labour administration to look for every opportunity to bury bad news on the last day of term and during the Christmas holiday.
"It gives the lie to Labour's spin that they are solving the asylum problem.
"Rather than requiring less facilities they are actually needing more and more and more facilities and taxpayers will be asking the legitimate question - where is this going to stop?"
The centre currently holds 150 men, women and children with extension bringing this up to almost 200.
The Scottish National Party's Westminster home affairs spokeswoman, Annabelle Ewing, said: "This expansion is entirely the wrong direction for Dungavel.
"Just as bad is the secretive way the government has announced this expansion."
A Home Office spokesman insisted the final decision to build the block had still to be taken but said some form of expansion would take place at Dungavel.
He added the extra space would only be used to accommodate single males and not families.
Sally Daghlian, chief executive of the Scottish Refugee Council, said the plans were "very disappointing".
"Asylum seekers are not criminals, they are people claiming protection from persecution under international law," she said.
"The government should stop holding children and reduce its use of detention overall to bring it into line with international standards rather than increasing the capacity of a centre which should only be used as a very last resort."