Britain has offered its "full support" to Lebanon following the shock resignation of Prime Minister Saad Hariri amid claims he is being held against his will in Saudi Arabia.
The Lebanese prime minister said on Sunday he would return to his country "very soon", in his first interview after quitting last week while in Riyadh.
Mr Hariri insisted he was "free" but said he decided to resign to save the country from imminent danger.
Lebanon fears it is being dragged into growing tensions between Sunni Saudi Arabia and Shia-led Iran.
Tehran backs the Shia Hezbollah group in Lebanon, which has a powerful military wing. It has been claimed Mr Hariri was being held because he failed to confront the organisation.
Foreign Secretary Boris Johnson warned Lebanon should not be used as a "tool for proxy conflicts".
He said: "I spoke today to foreign minister (Gebran) Bassil on the latest developments in Lebanon. I reiterated to him the United Kingdom's full support for the sovereignty and independence of Lebanon.
"Prime Minister Hariri has been a good and trusted partner for the UK, and we hope that he will return to Beirut without further delay, not least in the interests of Lebanon's political stability.
"We urge all parties with an interest in Lebanon to do all they can to encourage this and to work more broadly to reduce recent tensions.
"I reassured the foreign minister that the UK will continue to support the people and institutions of Lebanon at this challenging time.
"The Lebanese Armed Forces (LAF) is the only legitimate authority within Lebanon and no militia groups or foreign forces should aim to challenge the LAF as the ultimate guarantor of the Lebanese people's security.
"We echo the concerns of the United States and our European partners that Lebanon should not be used as a tool for proxy conflicts and that its independence and integrity should be respected by all parties - within the country and beyond."
Mr Hariri, a dual Lebanese-Saudi national, unexpectedly announced he was quitting in a clip broadcast on Saudi TV.