It said there were likely to be protests today which might result in violent clashes.
It has been advising against all travel to Yemen since March 4 and urging British nationals to leave immediately since March 12.
The Foreign Office said 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.”
Meanwhile, a radical Yemeni cleric says upheavals gripping the Arab world are an opportunity for al-Qaida fighters.
US-born Anwar al-Awlaki says the turmoil “opens the door for mujahedeen from all over the world” and claims it shows America has lost its allies in the region.
The cleric, believed to be hiding in largely lawless Yemen, says al-Qaida’s holy warriors are “going through a moment of elation” since the revolts that started in Tunisia and Egypt, and spread to Yemen, Bahrain and Syria as massive anti-government protests.
Al-Awlaki’s remarks were published in al-Qaida’s online magazine Inspire posted on militant websites.
The al-Qaida-linked cleric is believed to have offered inspiration to attacks in the United States.