An RAF reconnaissance aircraft will be sent to monitor the crisis in Ukraine, the Ministry of Defence has said.
A UK E-3D Sentry Airborne Warning and Control System (Awacs) jet will fly from RAF Waddington in Lincolnshire.
The aircraft would carry out its mission while staying within Polish and Romanian airspace, the MoD said.
Meanwhile, officials from several countries are due in London to discuss sanctions against Russian officials over its incursion into Ukraine.
David Cameron has said targeted sanctions could be introduced "within days" after Russian troops were deployed into the region of Crimea, in the south-east of the country.
Pro-Russian troops are still blockading Ukrainian troops across the autonomous region ahead of a secession referendum there on Sunday.'Alliance territory'Continue reading the main story
Reconnaissance planes will fly over Poland and Romania to monitor neighbouring Ukraine - not crossing into either Ukrainian or Russian airspace, said Nato.
Nato gave the go-ahead for the flights, which it said would "enhance the alliance's situational awareness", on Monday.
"All Awacs reconnaissance flights will take place solely over alliance territory," the official said.
An MoD spokeswoman said a single RAF plane and pilot would be involved in the mission.
"The North Atlantic Council on Monday decided to employ Awacs reconnaissance flights over Poland and Romania, as part of the Alliance's efforts to monitor the crisis in Ukraine," she said.
"We have said that the UK will meet all our commitments under the next level of Nato crisis response measures.
"As the UK E-3D Awacs is part of the Nato Awacs force, then it is only right that the UK undertakes its share of this mission."'At risk'
The US, Italy, France, Germany, Poland, Switzerland, Japan, Turkey and Canada are all expected to attend the talks in London later.
They will discuss measures including travel bans and asset freezes against 18 people linked to Vladimir Putin's government.
David Cameron has said Britain's own security and prosperity would be at risk if countries were able to flout international rules without facing consequences.
But he has said Russia still has an opportunity to resolve the situation diplomatically.
Moscow has officially denied that its troops are taking part in the blockades in Crimea but the government in Kiev - as well as the US and EU - accuse Russia of invading Ukraine, in violation of international law.
Unrest in Ukraine erupted in November after former President Viktor Yanukovych's last-minute rejection of a landmark EU deal in favour of a bailout from Russia.
Mr Yanukovych was ousted last month, and a new government has been voted in by the Ukrainian parliament which Russia says was a "coup".