Suspected criminals should not be automatically handed over on a European arrest warrant if there is a "real risk" they will suffer inhuman or degrading treatment in detention in the country where they are wanted, a ruling from the EU's top court suggests.
A warrant must at first be deferred and, if the risk cannot be discounted, a decision should be taken on whether the surrender procedure is brought to an end, the judgment said.
Britain is signed up to the EAW, which was introduced to speed up the extradition process between EU member states.
Keith Vaz, chairman of the Commons home affairs committee, said: "This ruling will further diminish the immediacy of the European arrest warrant which has already been beset by controversy.
"It is no longer the quick and effective remedy which it was intended to be."
He added: "It also has the potential to be used to delay prisoner transfer arrangements between EU countries which is costing the British taxpayer millions of pounds given the refusal of EU partners to take back their own citizens."
The Home Office said the ruling is consistent with domestic UK law.
A spokesman added: "The Extradition Act 2003 contains a number of safeguards, including grounds to refuse extradition where the judge considers that extradition is incompatible with the requested person's human rights."
The European Court of Justice (ECJ) was asked about the legal position following two cases in which warrants were issued in Hungary and Romania.
A Hungarian investigating magistrate issued two European arrest warrants with respect to Pal Aranyosi, a Hungarian national, so that a criminal prosecution could be brought for alleged offences of forced entry and theft.
The other case involved a European arrest warrant issued by a Romanian court with respect to Robert Caldararu to secure the enforcement of a prison sentence of one year and eight months imposed for driving without a driving licence.
Both men were located in Germany, where Mr Caldararu is currently detained. Mr Aranyosi is not in custody.
The Higher Regional Court of Bremen found that the detention conditions to which Mr Aranyosi and Mr Caldararu might be subject in the Hungarian and Romanian prisons respectively were contrary to fundamental rights which prohibit inhuman or degrading treatment or punishment.
Previously the European Court of Human Rights held that Romania and Hungary had infringed rights due to prison overcrowding.
The German court sought to ascertain from the ECJ whether in such circumstances the execution of arrest warrants can or must be refused, or made subject to a condition that information sufficient to establish detention conditions are compatible with fundamental rights is obtained.
The ECJ found that where an authority responsible for the execution of a warrant has evidence of a "real risk of inhuman or degrading treatment of persons detained in the member state where the warrant was issued" then it must assess that risk before deciding on the surrender of the individual concerned.
Where the risk derives from "general detention conditions", this cannot in itself lead to the warrant being refused.
It is necessary to demonstrate that there are substantial grounds for believing that the individual will be exposed to such a risk because of the conditions in which it is envisaged that he or she will be detained, the court found.
It said the authority responsible for executing the warrant must ask the issuing authority to urgently provide all the information necessary on the detention conditions in order to assess the risk.
A press release summarising the judgment concluded: "If, in the light of the information provided or any other information available to it, the authority responsible for the execution of the warrant finds that there is, for the individual who is the subject of the warrant, a real risk of inhuman or degrading treatment, the execution of the warrant must be deferred until there has been obtained additional information on the basis of which that risk can be discounted.
"If the existence of that risk cannot be discounted within a reasonable period, that authority must decide whether the surrender procedure should be brought to an end."
The ECJ does not decide the individual cases.