A Royal Navy ship will be sent to Beirut after the Lebanese city's port was destroyed by a huge explosion.
Defence Secretary Ben Wallace has authorised the deployment of survey vessel HMS Enterprise, following the blast which killed at least 135 people.
At least 5,000 people were injured and hundreds of thousands left homeless in the incident on Tuesday evening.
HMS Enterprise will carry out survey work in the port to establish the extent of the damage.
Mr Wallace said: "At the request of the Lebanese government, I have today authorised the sending of HMS Enterprise to help survey the Port of Beirut, assessing the damage and supporting the Lebanese government and people rebuild this vital piece of national infrastructure."
The Foreign Secretary meanwhile has promised a £5m support package for Lebanon.
The Ministry of Defence said the British military will "continue to work with the Lebanese government to help the people of Beirut recover".
"A small team of experts will deploy to the British Embassy in Beirut to help coordinate the UK response ahead of HMS Enterprise's deployment and a further package of support," the department added on Twitter.
The crew of HMS Enterprise will now spend their time alongside in Cyprus to get their survey motor boat ready to travel to Beirut.
Dominic Raab said he had spoken to Lebanon's Prime Minister Hassan Diab and pledged "to stand by the Lebanese people in their time of need".
Mr Raab said the support would include "search and rescue, humanitarian assistance up to £5m, as well as expert medical support".
Losses from the blast are estimated to be between $10 billion and $15 billion (£7.6bn to £11.4bn), Beirut governor Marwan Abboud told the Saudi-owned TV station Al-Hadath.
The blast is believed to have been triggered by a fire that touched off a giant quantity of ammonium nitrate fertiliser which has been stored in the port for several years.
The substance exploded with the force of a moderately strong earthquake.
Authorities have cordoned off the port itself, where the blast left a crater 200m (650ft) across and shredded a large grain silo, emptying its contents into the rubble.
Estimates suggested about 85% of the country’s grain was stored there and there are fears of imminent food shortages.