Volkswagen said that the crisis engulfing it was down to a “couple” of software developers in Germany as it admitted that alterations to vehicles may have an impact on their performance.

The company’s US chief executive issued a fresh apology yesterday and said that “defeat device” software in engines was installed with the express purpose of cheating emissions tests.

Addressing a US House of Representatives sub-committee, Michael Horn said that the company had been aware of “possible emissions non-compliance” in the spring of last year.

However, he insisted that the severity of the deception only became apparent on September 3 this year when warnings were made by US regulators.

He denied that there had been any “corporate decision” to install the devices, saying: “It was a couple of software engineers who put this in.”

Mr Horn admitted for the first time that the huge task of removing the devices from 11 million vehicles worldwide could damage their performance.

This will be seen as hugely significant as it could boost claims for a total refund by customers, dramatically escalating the scale of losses suffered by VW. Mr Horn’s comments were made on another day of drama for the carmaker as:

 •German police raided VW’s headquarters in Wolfsburg, as well as private apartments, and carried away files and hard disc drives as part of an investigation into the affair;

 •It was confirmed that more than ten senior managers, including three top engineers, had been suspended as part of an internal investigation. VW has also hired the US law firm Jones Day to conduct an inquiry.

 •Patrick McLoughlin, the transport secretary, said that tests have already started on diesel cars across Britain to establish real emissions levels. He called for an immediate EU-wide investigation into the use of “defeat devices” by other manufacturers.

•It was also confirmed that VW bosses in the UK would be called to appear before two separate parliamentary select committees next week.

Volkswagen has been embroiled in the deepest crisis in its 78-year history after revealing that it equipped 11 million diesel vehicles worldwide with software that switches the engine to a low-emissions mode during tests.

Appearing before the US energy and commerce committee yesterday, Mr Horn was asked if the devices had been installed to cheat tests. He said it “was installed for this purpose”.

“To my understanding, this was not a corporate decision; this was something individuals did,” he said.

Asked if he really believed that senior corporate managers had no knowledge of the software, Mr Horn replied: “I agree it’s very hard to believe, and I personally find it hard to believe.”

VW will begin recalling cars to have the software removed in January.

Mr Horn was asked whether “any fix is going to substantially change the cars”. He denied it would have an effect, but later added: “There might be a slight impact on the performance.”