One in 10 soldiers in the British army is from abroad, according to figures obtained by the Liberal Democrats.
Gurkhas are some of the foreign soldiers in the British Army
The party claimed the data showed citizens of 57 countries were being recruited to compensate for falling numbers of young Britons signing up.
Lib Dem defence spokesman Bob Russell said the reliance on foreign soldiers was "shocking", but was vital to maintain Britain's overstretched army.
The Ministry of Defence said current deployment levels were "sustainable".
Overseas recruits in the Army include those from Ghana and Ukraine.
Mr Russell, who represents the garrison town of Colchester, said: "For about 150 years the United Kingdom has recruited Gurkha soldiers from Nepal to serve in their own Gurkha regiments, and today there are around 3,000 of them.
"However, there are now nearly 6,700 solders from 57 other countries serving in British regiments - with Fiji leading the way with almost 2,000."
THE ARMY'S FOREIGN SOLDIERS
South Africa 720
St Vincent 280
St Lucia 225
Jamaica provides 975 soldiers, while there are 75 Australians serving in the Army.
Soldiers from non-Commonwealth countries, like France, Germany and Ukraine, are also represented.
Mr Russell said despite the foreign recruitment, the British army still had more than 1,000 vacancies.
"The British army is under-strength and over-stretched.
"The situation would be even worse if it was not for the fact that we have so many non- British citizens serving in our army."
The Lib Dem MP received the data about the forces after asking a written parliamentary question.
The MoD said the proportion of soldiers deployed had fallen over the past year to a "sustainable level".
A department spokeswoman said that employment in the Army was open to all Commonwealth citizens but they would have to meet the same "basic entry standards" as their British counterparts.
"We highly value the part played by Commonwealth soldiers in our armed forces," she said.