Dr Alan Billings, the PCC for South Yorkshire, has also raised concerns about delays in the Home Secretary announcing funding levels for police during the next financial year, also delayed as a result of escalating problems around Brexit.
During the life of the EU a network of agreements have been forged between police and security agencies in member countries, resulting in close links for tackling issues including organised crime and terrorism.
Dr Billings is now concerned that agreements made as part of the deal negotiated by Prime Minister Theresa MayÂ '“ while potentially less effective than current arrangementsÂ '“ may be lost if the country leaves under '˜hard Brexit' circumstances in March.
Four of the key agreements involving European countries are the Shengen Information System, which triggers alerts to law enforcement agencies across the continent about missing people, those wanted on warrant, those under discrete surveillance, and more.
That information can lead to the European arrest warrant protocol kicking in, which was responsible for 1,735 people being extradited from the UK to other EU countries last year, compared to less than 40 each year before the agreement was ratified.
The European criminal records information system allows electronic information to pass between countries without restriction and the UK is among the top three users of the system, with the Europol enquiry system also heavily used by UK agencies. From 150 Europol investigations this year, the UK has led on 25 per cent.
Being outside the EU would also cut the UK out of '˜Prum' arrangements, which are still being developed but which should lead to information such as DNA and fingerprints being electronically transferred, speeding up investigation work.
Dr Billings said Brexit without a deal in place would mean: 'You lose other deals which have been going on for 40 years, unless you can put something back.
'I think there are serious issues which would need to be addressed incredibly quickly.
'As I understand it, Mrs May's agreement includes a comprehensive security arrangement.
'It enables most of this to carry on, even if we are not at the table where all the rules are being made. Crime is increasingly international, terrorism likewise.
'In my view these agreements significantly tips the balance for law and order, against organised criminality and terrorism.
'I think police forces and the counter terrorism people have to make sure everything delivered through these is done in the best form it can be.
'It won't be 100 per cent but we need to get the best deal possible.'
The current situation created 'uncertainty and it is worrying', he said.
He also raised concerns about the shrinking opportunity to set next year's budget for South Yorkshire PoliceÂ because a Home Office announcement on how much forces nationally will be awarded from the GovernmentÂ '“ and what they may be allowed to raise locally as part of council taxÂ '“ have been stalled.
The delay originally was meant to allow this week's House of Commons vote to take place, but since that was cancelled it had become unclear what the new arrangements would be, he said.
Part of the process of setting a budget includes liaising with council leaders and holding public consultations, but those consultations could not take place until there was information to consult on, he said.