Almost 300,000 foreigners a year are allowed into Britain with wrongly approved visas.
Up to 15 per cent of short term visas issued by British embassies abroad are given in error, MPs were told.
Once here, migrants with dubious credentials are free to stay on illegally.
The visas are granted because it is easier for staff trying to hit processing targets to approve an application than reject one, explained Linda Costelloe Baker, who monitors Government visa refusals.
She told the Commons Home Affairs Committee there were errors in about 15 per cent of short-term visa approvals.
Officials were 'under pressure' to issue visas rather than reject the applications because of productivity targets, she said.