The Liberal Democrat leader - who is strongly in favour of an EU role in cross-border policing - protested that no deal had been reached in the coalition on the issue and insisted no announcements could be made by Tory ministers in Birmingham.
The Clegg veto is a sign of growing tension over EU policy as David Cameron comes under intense pressure from his party's anti-EU wing to claw back power from Brussels and grant a referendum on UK membership. But the deputy prime minister is determined to defend co-operation with the EU in areas where he believes it is performing well. , such as cross-border policing.
The Clegg veto is likely to be raised by Tory and Lib Dem MPs when home secretary Theresa May answers questions in the Commons tomorrow, the first day back after the autumn recess.
The Tory bid to announce a UK withdrawal has fuelled tension between the government and senior police and intelligence officers, who say that the move would harm international policing and mean the loss of UK influence over future European policy. It is understood that home secretary Theresa May, or possibly a cabinet colleague, had been lined up to make the announcement last Tuesday. But the plan was spotted by Danny Alexander, the Lib Dem chief secretary to the Treasury, who was given advance sight of Tory conference plans and speeches, leading to the veto.
The European arrest warrant (EAW), introduced in 2004, is the most high-profile EU instrument included in the proposed opt-out. It has been deployed in cross-border police efforts to track down and bring to justice paedophiles, international drug dealers and terrorists, including some involved in the 7/7 London bombings.
It is understood that, under complex EU procedural agreements, the Tories and Lib Dems may agree that the UK can initially withdraw from all the police and justice measures but then apply to rejoin individual elements such as the EAW at a later date. There would be no guarantee, however, that an application to rejoin would be allowed.
A decision on whether to opt out has to be taken by 2014 - at which point the 130 measures will fall under the full remit of the EU institutions, including the European court of justice and the European commission, rather than being administered on an intergovernmental basis.
The nightmare for Cameron could be that if, in order to appease the Lib Dems, he agrees to opt back in to parts of the police and justice package, his eurosceptics will then be able to say there should be a referendum, because UK powers will have been passed to the EU institutions under the new arrangements.
In a public letter earlier this year, more than 100 Tory MPs urged the government to pull out of EU police co-operation, including the EAW. "We do not wish to subordinate UK authorities to a pan-European public prosecutor. We do not want to see British police forces subjected to mandatory demands by European police under the European Investigation Order. We have deep concerns about the operation of the European Arrest Warrant for our citizens. We want the UK supreme court to have the last word on UK crime and policing, not the European court of justice," they said.
Senior police and intelligence officers working both in the UK and on the European mainland have lobbied ministers not to step back from cross-border policing. But Tory ministers say they will not allow the encroachment of the European court of justice into the UK legal and policing systems.
■ Tory sources said that chief whip Andrew Mitchell would be attending meetings in No 10 on Monday and would then return to "business as usual" in the Commons after the furore over his outburst at police guarding the entrance to Downing Street.
Bill Newton Dunn - Liberal Democrat MEP for East Midlands coments:
The UK needs better co-operation with neighbouring countries to defeat the criminals. We need to improve existing EU arrangements, not to opt out of them as the Conservatives and UKIP want. How on earth would opting out of co-operation against criminals help the UK to defeat them? The Conservative wish would be very damaging to the UK.