MI6 has warned that the war on Isis could last 50 years.
Boss Alex Younger said the recruitment drive is vital to keep Britain safe from extremist attacks if the war does last into the 2060s.
He said:“I would like to be optimistic about this but we have got quite long experience.
“Regrettably, this is an enduring issue which will certainly be with us, I believe, for our professional lifetime.”
The recruitment drive will see a 40% rise in spies to 3,500 by 2020.
He said fast changing technology “throws everything up in the air”.
MI6 – home to fictional spy James Bond, left – will hire whizzkids to keep ahead of terrorists.
Mr Younger added: “This issue will dictate our success.”
The MI6 chief also slammed whistle-blower Edward Snowden’s 2013 National Security Agency leak, calling it “highly problematic”.
Last year a government review said £2.5bn was being invested in security and intelligence staff.
It will ensure they have everything they need to “prevent and disrupt plots against this country at every stage”.