The founder of Microsoft has told the BBC there should be a debate about the degree to which tech firms co-operate with anti-terrorism investigations.

Bill Gates made his comments in response to a question about Apple's refusal to help the FBI unlock an iPhone used by Syed Rizwan Farook who, along with his wife, killed 14 people and injured 22 others in California last December.

In a separate interview with the Financial Times Mr Gates appeared to go further, contradicting Apple's chief executive Tim Cook's suggestion that it would set a "dangerous precedent".

However, he did not explicitly say Apple should comply.