Bulgaria and Romania joined the European Union "before they were ready" a group of peers has said, as the coalition and Labour attempt to deal with public anxiety about immigration.
In a report published today, the House of Lords EU committee said the union needed to ensure countries wishing to become members were committed to democracy, human rights and the rule of law.
However the peers said these tests were not applied rigorously enough when it came to accepting Romania and Bulgaria in the "scramble" to allow them to join in 2007, leading to the "unsatisfactory" situation of tests having to be applied after the fact.
"Progress still needs to be made on judicial reform and corruption in Romania and Bulgaria, as well as on fighting organised crime in Bulgaria," the peers said.
"The last six years indicate that post-accession conditionality achieves only slow progress. In future accessions, every effort must be made to ensure that all reforms are irreversible prior to accession, as post-accession mechanisms are both undesirable and unlikely to prove effective."
The peers also said it was a "grave mistake" to allow Cyprus to join the EU before the argument with Turkey over the north of the island was settled as it meant Brussels was now in dispute with a candidate country.
The report comes as transition controls on internal EU migration are set to be lifted for Romania and Bulgaria. From next year workers from those two countries will have the same rights of freedom of movement as other EU nationals.
On Tuesday work and pensions secretary Iain Duncan Smith said there was a "crisis" over the issue as he feared "some people want to come here solely to claim benefits".
And today Labour leader Ed Miliband called for minimum wage rules to be more strictly enforced as a method of cracking down on low-skilled immigration and for "maximum transitional controls for new countries coming in from Eastern Europe".
The surge in the Ukip vote at last week's Eastleigh by-election has focused the minds of the main three political parties on how to deal with perceived public concern over migration.