All British nationals have been urged to leave Yemen in the run-up to likely protests which could result in violent clashes.
The Foreign Office said people should get out while commercial airlines are still flying, "in light of the rapid deterioration in the security situation".
It warned that it was highly unlikely it would be able to evacuate British nationals in the event of increased disorder.
It said in a statement: "In light of the rapid deterioration in the security situation in Yemen and likely protests on Friday April 1 which might result in violent clashes, we strongly urge all British nationals to leave the country now while commercial airlines are still flying.
"Given the situation on the ground, it is highly unlikely that the British Government will be able to evacuate British nationals or provide consular assistance in the event of a further breakdown of law and order and increased violent civil disorder. British nationals should therefore plan accordingly.
"We have advised against all travel to Yemen since March 4 and have urged British nationals to leave immediately since March 12. The latest amendment to our travel advice reflects the increasing seriousness of the situation.
"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: without this trust, no agreement can be reached. The Yemeni people want to see their legitimate demands acknowledged and met and the UK fully supports them in this aspiration."