The move ramps up efforts to identify and compensate those on the controversial list. According to the ICO, only 467 people have been provided with copies of the information that was held on them.
Blacklisting was discovered in 2009 when thousands of names, mainly construction workers, were found on a list held by The Consulting Association (TCA) when its offices were raided by the Government's data watchdog.
The TCA was set up in 1993 with funding from construction firm Sir Robert McAlpine. Several large construction and engineering firms colluded in the TCA's formation, according to evidence presented to the Scottih Affairs Committee. Construction companies pay a fee to access to TCA data, which includes blacklisted workers who are thought to be 'militant' or 'troublemakers'.
Unions claim blacklisted workers have been denied employment, often for raising health and safety issues or for being union activists.
"We're asking those people to reply to us and provide some form of identification if they would like us to check whether the information included on the list is about them," said David Smith, deputy commissioner and director of data protection.
"If it is, then they will receive a copy of their information. How much detail they'll receive in response to their enquiry will vary in each case."
Who's to blame?
Construction companies, including Balfour Betty, Carillion and Sir Robert McAlpine have denied legal liability, but said they would offer to pay up to £10,000 to victims of blacklisting.
Unions said this does not go far enough and have called for an admission of legal liability and a public inquiry.
Maria Ludkin, GMB national officer for legal and corporate affairs, said: "GMB is pleased that after almost two years of pressure ICO has bowed to the inevitable and is writing to 1,200 people whose national insurance number states that they are on the blacklist."
At present, 467 out of 3,214 workers have been made aware they were on the blacklist. Once 1,200 more have been contacted, there 1,546 left to trace.
Earlier this week, Labour leader Ed Miliband called for a fresh inquiry into blacklisting. "Trade unionists and other campaigners have worked tirelessly to keep this scandal in the public eye," he said.
"Blacklisting is about a race to the bottom: lower standards, insecurity at work, fewer rights and worse conditions.
"My message is that the Government must now end its refusal to act and hold the inquiry into blacklisting that common sense and decency demand."
