MI6 is to recruit 1,000 new spies following a stark warning from its chief that the threat from Islamic State-style terrorism will last a 'professional lifetime'.

Alex Younger – known as 'C' – said IS-style terrorism posed a 'persistent threat', driven by the internet revolution and the breaking down of international barriers.

In a rare public appearance at a security conference in Washington DC, he said 'deep social economic and demographic drivers' meant there was little sign of the 'enduring' danger disappearing soon.

MI6 is to recruit 1,000 new spies following a stark warning from its chief that the threat from Islamic State-style terrorism will last a 'professional lifetime'. Alex Younger – known as 'C' – said IS-style terrorism posed a 'persistent threat', driven by the internet revolution and the breaking down of international barriers

And he warned of the damage done by the theft and leaking of US and UK secrets by Edward Snowden.

His warning came as it emerged that MI6, the Secret Intelligence Service, is to dramatically expand its staff with another 1,000 spies – a 40 per cent rise by 2020 to 3,500.

The expansion was reported by BBC Newsnight, which suggested the new staff were needed because technology, including the internet and facial-recognition techniques, had made it easier for the UK's enemies to identify intelligence officers.

The staff expansion follows a string of terrorist attacks on continental Europe over the past 12 months, including the November 2015 attack on Paris, the worst in its history, and the July attack in Nice (above) when a lorry mowed down crowds celebrating Bastille Day

It follows a string of terrorist attacks on continental Europe over the past 12 months, including the November 2015 attack on Paris, the worst in its history, and the July attack in Nice when a lorry mowed down crowds celebrating Bastille Day.

In March 32 people were killed in twin suicide attacks in Brussels.

Mr Younger did not mention the rise in staff numbers, but did warn of the damage done by the leaking of secrets by Snowden, and said developments in technology presented both an 'existential threat and a golden opportunity' to security agencies.

Mr Younger's comments were shown on a recording released by the Centre for Cyber and Homeland Security at the George Washington University.

Mr Younger said the 2013 National Security Agency leak by Edward Snowden (above), who later claimed asylum in Russia, had been 'highly problematic'

Asked if the terror threat from groups such as IS and Al Qaeda had reached its peak, he said: 'I would like to be optimistic about this but we have got quite long experience of this phenomena now and I see it very much as the flipside to some very deep-seated global trends, not least of all globalisation – the reduction of barriers between us.

'It's a function also of the information revolution and the capacity for ideas to travel. It is fuelled by a deepening sectarian divide in the Middle East and there are some deep social economic and demographic drivers to the phenomenon that we know as terrorism.

'Allied with the emergence of state failure this means that, regrettably, this is an enduring issue which will certainly be with us, I believe, for our professional lifetime.'

Mr Younger, who became chief of the Secret Intelligence Service in November 2014, said terror groups took 'ideological succour' from gaining territory and it was 'good' that IS were being pushed back.

'But I regret that I would have to forecast that whilst it is wholly desirable to remove territory, you will have a persistent threat representing some of the deep fault lines that still exist in our world that I think will be addressed by patience, long-term partnership and partnership that esteems the collective values that we defend,' he said.

Mr Younger said 'deep social economic and demographic drivers' meant there was little sign of the 'enduring' danger disappearing soon. Pictured, flowers outside the Bataclan theatre in Paris after the terrorist attacks of 2015

He said rapid technological changes 'throw everything up in the air' and mean the service has to 'fundamentally' look at the way it carries out intelligence operations.

Mr Younger said the 2013 National Security Agency leak by Snowden, who later claimed asylum in Russia, had been 'highly problematic'.

'The real issue for us has been the effect that this has had on the levels of trust between the intelligence communities internationally and the technology community, where I think that the right and proper response to the common threats that face us is through community of effort and teamwork between those different groups,' he said.

'And to the extent that those revelations damaged and undermined the trust that needs to exist, I think it is highly problematic.'

A report by MPs this week warned the war against Islamic State – also known as Daesh – could take more than 50 years.

The Commons defence committee also warned IS appeared to be 'putting down roots' in Afghanistan, was training Libyan extremists as they spread along the North African coast and strengthening ties with Boko Haram in Nigeria.