MI6 Chief Richard Moore said China, Russia, Iran and international terrorism were the U.K.'s "big four" security issues, with China taking the No. 1 spot.
Sir Richard Dearlove said it would likely be "difficult to prove" that the Wuhan lab in China was behind the leak.
The president has threatened to declassify documents related to the Justice Department's probe into Russia election interference.
"Between us, we have spent close to 90 years imprisoned without charge or trial," the letter says.
Convicted Russian spy Sergei Skripal's "substance exposure" is drawing comparisons to the 2006 death of Russian intelligence agent Alexander Litvinenko's acute radiation poisoning.
A Spanish double agent did everything he could to stop his wife from blowing his cover.
International cyber crime groups are advancing faster than agencies can put up defenses.
John Sawers and Jonathan Evans write that a withdrawal vote would hurt the EU and leave Britain vulnerable.
Former CAPO agent Uno Puusepp has been unveiled as a double agent for Russia's FSB
Britain's security services have for years been accused of colluding in the ill-treatment of suspected militants.