Britain is to build embassies in up to five more countries but close dozens of other diplomatic outposts as part of a major cost-cutting drive, William Hague is set to announce.
The Foreign Secretary is to give MPs more details of how he hopes to slash around £100 million from costs by 2013 while ensuring "no strategic shrinkage of Britain's influence".
El Salvador, Kyrgyzstan and the newly formed republic of South Sudan could all have embassies by 2015 and the UK diplomatic presence in China and India will be deepened.
There are also plans to introduce representation to Madagascar and Somalia, although without improvements in their political situation, security costs would be prohibitive, a source said.
The expansion comes at the same time as significant reductions elsewhere, with Europe set to bear the brunt of closures and sell-offs of consulates and other satellite offices.
Many are housed in luxury properties in prime city locations which are expected to raise up to £200 million when they are sold off - on top of £34 million saved in estate and security costs. A Foreign Office source said many of their functions were no longer required and that it would help counter criticism of staff "living it up in luxury accommodation".
British tourists would still be able to access help while abroad, the source added.
Another £30 million in savings is being sought in "back office" and administration functions with a move to using more local staff and cheaper contracts for work such as cleaning. A similar saving is being sought from the wage bill, with hundreds of overseas postings being axed and diplomatic staff placed in cheaper UK-based jobs.
Mr Hague, who will set out details of the changes to MPs in a Commons statement, said the package represented "the biggest strategic diplomatic advance by Britain in decades".
"As Foreign Secretary I announced on day one that my objective is to ensure that there is no strategic shrinkage of Britain's influence under this government," he said. "Our diplomatic network is the essential infrastructure for our foreign policy and our influence overseas. Taken together, this represents the biggest strategic diplomatic advance by Britain in decades. So by 2015 we must aim to be a Foreign Office that is lean and efficient but configured to match the realities of the 21st century."