Figures released on Bulgarian and Romanian migrant workers
The Office for National Statistics (ONS) has for the first time published a breakdown of Romanians and Bulgarians in employment, an annual rise of nearly 15%.
Bulgarians and Romanians will this year gain the same rights to work in the UK as other EU citizens.
This has sparked concerns about a rise in migration from the two countries.
In response to the end of the temporary work restrictions, the UK government is considering limiting migrants access to benefits, healthcare and housing.
The ONS published its regular employment statistics with a new breakdown of data showing where foreign workers were from - with those originating in recent European Union (EU) accession countries such as Poland in a separate category to more established nations such as France and Spain.
It also separated out workers from Romanian and Bulgarians in response to "customer demand".
The figures showed the number of migrants from the two countries already working in the UK had risen from 90,000 in the first three months of 2012 to 103,000 in the first three months of 2013.
A British Labour Force survey has previously calculated that there are currently 26,000 Bulgarians and 80,000 Romanians living in the UK.
Bulgarians and Romanians have had the right to visa-free travel to the UK since 2007, when their countries joined the EU, but there have been temporary restrictions on the kind of jobs they could take.
Restrictions being eased
These restrictions will be dropped at the end of the year in accordance with EU rules.
The UK government has declined to give estimates of how many migrants it expects but Migration Watch, which campaigns for tighter controls of immigration, has said it could be anywhere between 30,000 and 70,000.
Diplomats from Bulgaria and Romania have repeatedly stressed they are not expecting a "wave" of migration from their countries.
Heather Rolfe, principal research fellow at the National Institute of Economic and Social Research, said the figures were broadly in line with the institute's previous estimates and they showed that EU migrants travel in order to find work.
She told the BBC: "Migrants want to improve their lives.
"The fact they (Romanians and Bulgarians) have come here given all the restrictions and found work is really a positive for the UK.
"It shows people who are coming really want to contribute to the economy and are contributing to the economy."