As many as 14 million current and former civilian US government employees had their personal information exposed to hackers, according to two people who were briefed on the investigation, representing a far higher figure than the 4 million the Obama administration initially disclosed – and coming amid an apparent second cyber breach that officials said was linked to China.

The newer estimates put the number of compromised records at between 9 million and 14 million going back to the 1980s, said one congressional official and one former US official, who spoke to the Associated Press on condition of anonymity because information disclosed in the confidential briefings includes classified details of the investigation.

There are about 4.2 million federal employees, so the majority of the records exposed relate to former employees. Contractor information also has been stolen, officials said.

The latest revelation came a day after a major union said the cyber theft is more damaging than it first appeared, asserting that hackers stole personnel data and social security numbers for all the federal workers in a central personnel database.

Several US officials also said hackers linked to China appear to have gained access to sensitive background information submitted by intelligence and military personnel seeking security clearances. The break is the second digital breach of federal records revealed in a week and could dramatically compound the potential damage.

The forms believed accessed, known as Standard Form 86, require applicants to fill out deeply personal information about mental illnesses, drug and alcohol use, past arrests and bankruptcies. They also require the listing of contacts and relatives, potentially exposing any foreign relatives of US intelligence employees to coercion. The applicant’s social security number and that of his or her cohabitant are required.

The officials declined to be quoted by name because the details are classified, and the Office of Personnel Management had no immediate comment.

Regarding the new user total affected by the main breach, the Obama administration had acknowledged that up to 4 million current and former employees whose information resides in the OPM server are affected by the December cyber breach, but it had been vague about exactly what was taken.

But J David Cox, president of the American Federation of Government Employees, said in a letter Thursday to OPM director Katherine Archuleta that based on incomplete information OPM provided to the union, “we believe that the Central Personnel Data File was the targeted database, and that the hackers are now in possession of all personnel data for every federal employee, every federal retiree, and up to 1 million former federal employees”.

The OPM data file contains the records of most federal civilian employees, though not members of Congress and their staffs, members of the military or staff of the intelligence agencies.

The union believes the hackers stole military records and veterans’ status information, address, birth date, job and pay history, health insurance, life insurance and pension information, and age, gender and race data, he said.

Also Thursday, Senate Democrat leader Harry Reid said that the hack was carried out by “the Chinese” without specifying whether he meant the Chinese government or individuals. Reid is one of eight lawmakers briefed on the most secret intelligence information. US officials have declined to publicly blame China, which has denied involvement.

The union, which does not have direct access to the investigation, said it is basing its assessment on “sketchy” information provided by OPM. The agency has sought to downplay the damage, saying what was taken “could include” personnel file information such as social security numbers and birth dates.

“We believe that social security numbers were not encrypted, a cybersecurity failure that is absolutely indefensible and outrageous,” Cox said in the letter. The union called the breach “an abysmal failure on the part of the agency to guard data that has been entrusted to it by the federal workforce”.

Samuel Schumach, an OPM spokesman, said that “for security reasons, we will not discuss specifics of the information that might have been compromised”.

Schumach did, however, address Cox’s comment on encryption. “Though data encryption is a valuable protection method, today’s adversaries are sophisticated enough that encryption alone does not guarantee protection,” he said. “OPM does utilize encryption in some instances and is currently increasing the types of methods utilized to encrypt data.”

The central personnel data file contains up to 780 separate pieces of information about an employee.

Cox complained in the letter that “very little substantive information has been shared with us, despite the fact that we represent more than 670,000 federal employees in departments and agencies throughout the executive branch”.

The union’s release and Reid’s comment in the Senate put into sharper focus what is looking like a massive cyber espionage success by China. Republican senator Susan Collins, an intelligence committee member, has also said the hack came from China.

Mike Rogers, the former chairman of the House intelligence committee, said last week that Chinese intelligence agencies have for some time been seeking to assemble a database of information about Americans. Those personal details can be used for blackmail, or also to shape bogus emails designed to appear legitimate while injecting spyware on the networks of government agencies or businesses Chinese hackers are trying to penetrate.

US intelligence officials say China, like the US, spies for national security advantage. Unlike the US, they say, China also engages in large-scale theft of corporate secrets for the benefit of state-sponsored enterprises that compete with Western companies. Nearly every major US company has been hacked from China, they say.