The Foreign Office has urged British nationals to leave Yemen while commercial airlines are still flying, “in light of the rapid deterioration in the security situation”.
Protests in recent weeks have brought president’s Ali Abdullah Saleh’s 32-year-old rule to the verge of collapse.
Updated Foreign Office travel advice says there is a “high possibility of violent demonstrations” on Friday.
It said the UK may be “unlikely” to be able to evacuate citizens in future.
Officials have been advising against all travel to Yemen since 4 March and urging British nationals to leave immediately since 12 March.
President Saleh has agreed to resign by January 2012, but the opposition and protesters are calling for his immediate departure.
The Foreign Office advice reads: “A rapid deterioration in the security situation is possible if negotiations between the president and the opposition breakdown.
“We call on all parties to make urgent progress in implementing much needed political and economic reform”
Foreign Office
“Routes in and out of Sanaa and the other major cities may be blocked and airports closed or inaccessible.”
In a statement, the Foreign Office said it was also “highly unlikely” to be able provide consular assistance in the event of a further breakdown of law and order.
It added: “We urge all parties in Yemen to exercise the utmost restraint and take all steps necessary to defuse tension on the ground.
“We call on all parties to make urgent progress in implementing much needed political and economic reform. The government of Yemen must take urgent action to build trust with the opposition and with the protesters.”
This article is from the BBC News website. © British Broadcasting Corporation, The BBC is not responsible for the content of external internet sites.