Britain's government opted out of all 133 EU police and criminal justice measures last year, but decided to rejoin some of them before that decision came into effect today.
Parliament voted to back that decision in November.
The decision came despite growing questions over Britain's place in the EU, with Prime Minister David Cameron promising a referendum on its membership in 2017.
"The European Commission very much welcomes the move by the UK, which means that the UK will continue to work alongside its neighbours to fight against cross-border crime in an EU-wide framework," the commission's First Vice President Frans Timmermans said.
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The European Arrest Warrant was introduced in 2004 to streamline extradition proceedings in the wake of the September 11, 2001, attacks on the United States.
It has been used in high-profile cases including that of WikiLeaks founder Julian Assange, who has sought refuge in Ecuador's embassy in London to avoid extradition to Sweden to face questioning over allegations of sexual assault.
Britain won the right for the original opt-out during negotiations with Brussels over the Lisbon Treaty, the document that laid the basis for the current shape of the EU, in 2009.
In a speech on Friday he promised tough curbs on welfare for EU migrants, warning that they would require changes to European treaties but that Britain's EU membership could be in doubt if other European leaders refused.
Britain has won several major EU opt-outs since joining the EU in 1973, staying out of the euro currency, the Schengen travel area, parts of the EU rights charter and parts of justice legislation.