Next year transitional controls placed on immigration from the two nations, which joined the EU in 2007, will be lifted, allowing Romanians and Bulgarians the same rights of free movement enjoyed by others in the union.
Immigration minister Mark Harper was repeatedly questioned on the issue by Conservatie MPs in the Commons on Monday, many of whom fear a "surge" in migration.
Bury St Edmunds MP David Ruffley said Britain should consider taking new powers to curb "benefit tourism undertaken by Romanians and Bulgarians". He said such "welfare tourism" added to British public spending rather than reduced it.
And Basildon and Billericay MP John Baron said the fact the national minimum wage in the UK was "five or six times" higher than in Bulgaria and Romania could fuel a "surge" in mass migration.
Harper told his Conservative backbenches that the government had not made "speculative projections" about how many Romanians and Bulgarians would want to move to the UK as this could not be done with any degree of accuracy.
"We want to make sure when people look at access to our benefits and our services no one thinks we are a soft touch," he said.
Earlier this month leading politicians from both Romania and Bulgaria said Britain was not first choice for their citizens who wished to move.
Bulgarian foreign minister Nikolay Mladenov said many of his compatriots would much rather travel to Germany, Spain and Italy, as his country has stronger business links with those countries.
In January it was reported the British government was considering running adverts to dissuade would-be immigrants from moving to the UK.
The rumour prompted a Romanian newspaper to hit back by posting adverts encouraging Britons to move to Romania instead. The adverts included the claim that "Half of our women look like Kate. The other half like her sister".
The debate over immigration comes as the Romanian prime minister strongly denied allegations that abattoirs in his country were the source of horse meat found it beef products sold across the EU.