The arrests of the 42-year-old woman and 60-year-old man were made after searches at addresses in north and north-west London on 24 June, the Metropolitan police said.
A third property was also searched in north-west London on 9 September after their arrest under the National Security Act (NSA).
The investigation is not linked to attacks at buildings and venues linked to Jewish and Iranian communities in London, the force added.
It is also not thought to be linked to any activity or events linked to the Jewish high holy days period.
The man and woman have been released on conditional bail until a date in October.
Cmdr Helen Flanagan, the head of counter-terrorism policing London, said: “We know that news of these arrests will come as a concern to some members of the public, but I want to stress that we do not believe there to be any imminent threat to the wider public in connection with this matter.
“We’ve seen a marked increase in the tempo of our work linked to national security and state threats over the past year and this investigation is a continuation of that.
“As we’ve shown here, if we suspect any activity could threaten the safety of the UK or the public, then we will not hesitate to take action.
“I would also like to remind the public that the overall terrorism threat level is at severe, so we continue to ask for everyone’s support through being vigilant, and if you see or hear anything that doesn’t look right, then please, trust your instincts, and report it to us.”
