The foreign secretary has travelled to Tunisia just weeks after popular protests brought down the previous government.
William Hague will meet senior ministers in the interim government.
He will go on to visit more nations in north Africa and the Middle East, and will announce new funding for projects linked to political and social reform.
But he will not travel to Egypt where protests demanding the resignation of President Hosni Mubarak are continuing.
Tunisia’s President Zine al-Abidine Ben Ali resigned last month following weeks of protests against his 23-year rule.
His departure sparked a wave of similar protests elsewhere in the Arab world, most prominently in Egypt, where up to 300 people are thought to have been killed in two weeks of demonstrations against Mr Mubarak.
Unrest has also been seen in Jordan, where King Abdullah II dismissed his government, and in Yemen, where President Ali Abdullah Saleh announced he would not stand for re-election.
‘Political openness’
The Foreign Office has not revealed which four countries Mr Hague will visit after Tunisia.
But a spokesman described the current period as “a time of great opportunity in the region”, and said the foreign secretary’s discussions would focus on “calls for greater political openness and economic development in the Middle East in the light of recent events in Tunisia and Egypt”.
While in Tunisia, Mr Hague is due to meet leading figures from the interim government including Prime Minister Mohammed Ghannouchi and International Co-operation Minister Mohamed Nouri Jouini.
The interim government has promised to hold fresh elections within six months and Mr Ghannouchi – who served for many years in the ousted president’s regime – has said he will retire once they are complete.
Mr Hague will also announce new UK funding for projects aimed at boosting access to justice, freedom of expression, democratic institutions and civil society.
On Monday night, he posted a message on micro-blogging website Twitter, saying: “Heading to Tunisia to meet the new interim government & show UK support for the people of Tunisia & their democratic hopes.”
BBC diplomatic correspondent James Robbins said Egypt was not on the three-day itinerary because the UK government did not want to be accused of trying to dictate precisely how the country should be transformed.