Russia was an immediate problem, but China poses the greatest long-term challenge, the head of MI5 intelligence agency said on Wednesday.
Speaking to reporters, Ken McCallum said that "hostile states" no longer simply steal secrets and spy, but also target the UK's democracy, economy, and infrastructure.
"If the question is - which countries' intelligence services cause the most aggravation to the UK in October 2020 the answer is Russia," McCallum said.
"If on the other hand the question is which state will be shaping our world across the next decade, presenting big opportunities and big challenges for the UK the answer is China. You might think in terms of the Russian intelligence services providing bursts of bad weather, while China is changing the climate."
He also spoke of the threat from terrorism.
He said that eight of the 27 terrorist attacks that have been disrupted at a late stage since 2017 were by far-right extremists.
He said the agency operated the same system as on other forms of extremist terrorism, "with cases pursued by the same counter-terrorist professionals within the same part of the MI5/police machine, operating to the same thresholds, prioritized on the basis of threat and risk rather than ideology."
McCallum said the threat of far-right terrorism is growing.
"Those threats are becoming more diverse and in some ways harder to spot. We face a bit of a nasty mix - terrorism isn't going away, and state-backed hostile activity is on the rise. But let's not make these people 10 feet tall," he said.
"Based on hard experience, the UK has a mature national security system, MI5 is a capable member within a genuine team. Every day that team includes support from individual members of the public."
He said he was speaking to journalists in an effort to make the spy agency more open.
"Much of what we do needs to remain invisible, but what we are doesn't. In fact, opening up in the right ways is key to our future success," McCallum said.
"Our increasingly diverse people, our partnerships in all sorts of places, and advances in technology give us reason to be confident. You will know when we're succeeding - as nothing will hit the headlines."