Takata Corp. has agreed to declare 33.8 million airbags defective, a move that will double the number of cars and trucks included in what is now the largest auto recall in US automotive history affecting models from 11 carmakers.

The number is double previous estimates for faulty airbags from the manufacturer.

US regulators said Takata has still not found the cause of the defects.

The airbags have been linked to six deaths and more than 100 injuries.

“Today is a major step forward for public safety,” US Transportation Secretary Anthony Foxx said.

“The Department of Transportation is taking the proactive steps necessary to ensure that defective inflators are replaced with safe ones as quickly as possible, and that the highest risks are addressed first.”

The Transport Department’s National Highway Traffic Safety Administration said its analysis of test results “points to moisture infiltrating the defective inflators over extended periods of time as a factor”.

That moisture may make the chemicals that ignite to set off an airbag burn too quickly, causing the structure to break and “sends metal shards into the passenger cabin that can lead to serious injury or death”, said the administration.

Both passenger and driver airbags will be recalled in an effort that started in high-humidity areas of the US, but will now be national.

Last week, Honda and Daihatsu said they would recall some 5 million cars globally to replace the potentially deadly airbag inflators made by Japanese air bag maker Takata.

Toyota and Nissan also said they would be recalling 6.5 million vehicles over the same issue.

Honda said that the models affected included the Fit subcompact and would not affect its cars sold in the US, where most of the deaths occurred.