Theresa May has been warned today that the measure she hopes will bring to justice the Russian spies blamed for the Salisbury poisonings is set to be lost after Brexit.

The European Union is on course to reject the prime minister’s call for a unique deal to preserve existing security cooperation, including through the European Arrest Warrant (EAW), a study concludes.

The alarm is raised the day after Ms May announced an arrest warrant had been obtained to seize the Moscow intelligence officers suspected of being behind the attack on Sergei Skripal and his daughter.

The UK extradites more than 150 suspects a year from other EU countries and sends back around 1,100 – compared with less than 60 before the EAW was introduced.

Now the report, by the Institute for Government (IfG) think tank, has warned the EU is offering only a “slightly” better security deal than with other non-EU countries – far short of Ms May’s dream of a “deep and special partnership”.

“A deal based on precedent would be better than no deal at all, but it would still be damaging,” its report said.

“The countries with the closest security cooperation with the EU, including Norway and Switzerland, do not have access to all EU databases, cannot participate fully in the operations of Europol, the EU’s police agency, and have more complicated extradition arrangements with the EU.”

Without the EAW, the extradition of foreign criminals would be “slower and more bureaucratic”, with suspects enjoying more “procedural rights”, the IfG said.

Police chiefs in Northern Ireland, who “treasured” the EAW, feared a return to the “almost toxic relationship” with Dublin during the Troubles, which meant extraditions “could take years”.

The gloomy conclusions come after Amber Rudd, the former home secretary, warned Ms May, in the Commons, that any loss of security cooperation would be “completely unacceptable to the people of the UK”.

The biggest issues facing UK on leaving EU Show all 8 1 /8 The biggest issues facing UK on leaving EU The biggest issues facing UK on leaving EU Post-Brexit immigration workers sorting radishes on a production line at a farm in Norfolk. One possible post-Brexit immigration scheme could struggle to channel workers towards less attractive roles - while another may heighten the risk of labour exploitation, a new report warns. PA The biggest issues facing UK on leaving EU Customs union A key point in the negotiations remains Britain's access to, or withdrawal from, the EU customs union. Since the referendum there has been hot debate over the meaning of Brexit: would it entail a full withdrawal from the existing agreement, known as hard Brexit, or the soft version in which we would remain part of a common customs area for most goods, as Turkey does? No 10 has so far insisted that “Brexit means Brexit” and that Britain will be leaving the customs union, but may be inclined to change its position once the potential risks to the UK’s economic outlook become clearer. Alamy The biggest issues facing UK on leaving EU Northern Ireland-Irish border Though progress was made last year, there has still been no solid agreement on whether there should be a hard border between Northern Ireland and the Republic of Ireland. To ensure borderless travel on the island, the countries must be in regulatory alignment and therefore adhere to the same rules as the customs union. In December, the Conservative Party’s coalition partners, the DUP, refused a draft agreement that would place the UK/EU border in the Irish Sea due to its potential to undermine the union. May has promised that would not be the case and has suggested that a “specific solution” would need to be found. Getty The biggest issues facing UK on leaving EU Transition period Despite protests from a small number of Conservative MPs, the Government and the EU are largely in agreement that a transitional period is needed after Brexit. The talks, however, have reached an impasse. Though May has agreed that the UK will continue to contribute to the EU budget until 2021, the PM wants to be able to select which laws made during this time the UK will have to adhere to. Chief negotiator Michel Barnier has said the UK must adopt all of the laws passed during the transition, without any input from British ministers or MEPs. EPA The biggest issues facing UK on leaving EU Rights of EU citizens living the UK The Prime Minister has promised EU citizens already living in the UK the right to live and work here after Brexit, but the rights of those who arrive after Brexit day remains unclear. May insists that those who arrive during the transition period should not be allowed to stay, whereas the EU believe the cut-off point should be later. Getty The biggest issues facing UK on leaving EU Future trade agreement (with the EU) Despite this being a key issue in negotiations, the Government has yet to lay out exactly what it wants from a trade deal with the EU. Infighting within the Cabinet has prevented a solid position from being reached, with some MPs content that "no deal is better than a bad deal" while others rally behind single market access. The EU has already confirmed that access to the single market would be impossible without the UK remaining in the customs union. Getty The biggest issues facing UK on leaving EU Future trade agreements (internationally) The Government has already begun trying to woo foreign leaders into prospective trade agreements, with various high profile state visits to China, India and Canada for May, and the now infamous invitation to US President Donald Trump to visit London. However the UK cannot make trade agreements with another country while it is still a member of the EU, and the potential loss of trade with the world's major powers is a source of anxiety for the PM. The EU has said the UK cannot secure trade deals during the transition period. EPA The biggest issues facing UK on leaving EU Financial services Banks in the UK will be hit hard regardless of the Brexit outcome. The EU has refused to give British banks passporting rights to trade within the EU, dashing hopes of a special City deal. However according to new reports Germany has suggested allowing trade on the condition that the UK continues paying into the EU budget even after the transition period. Getty

And they come despite the prime minister effectively abandoning her refusal to accept the jurisdiction of the European Court of Justice (ECJ) after Brexit, one major obstacle to a security deal.

The white paper that followed the Chequers deal “accepted the sole competence of the ECJ in interpreting EU rules”, the IfG noted – prompting Michel Barnier, the EU’s Brexit negotiator, to speak more warmly of a security deal.

However, weak data protection rules remained a huge stumbling block, with the ECJ having twice struck down the UK’s “handling of personal data”.

This year’s Data Protection Act “could also cause problems”, the report said, partly because it removed data protection rights for any investigations relating to immigration probes.

Tim Durrant, the IfG’s senior researcher, warned: “Both sides will have to move to avoid a serious reduction in cooperation which would only benefit criminals.”

Naming the Salisbury suspects on Wednesday, the prime minister told MPs: “We have obtained a European Arrest Warrant and will shortly issue an Interpol red notice.”

“Should either of these individuals ever again travel outside Russia, we will take every possible step to detain them, to extradite them and to bring them to face justice here in the United Kingdom.”