Fighting terrorism and people trafficking will be more difficult after Brexit, the EU’s security commissioner has warned MPs.

Sir Julian King, who is British, said there would be “practical limits” on sharing crime-fighting information, even if the post-withdrawal negotiations go well.

Speaking to a Parliamentary inquiry, Sir Julian hailed the “material contribution to the UK’s security” from the EU’s hard-earned cross-border security agreements.

He acknowledged both Theresa May and the EU had a “shared interest” in trying to reach new arrangements that could deliver the same benefits, after Brexit.

But, on efforts to replicate Europol – the EU-wide law enforcement agency – Sir Julian warned: “There will be some legal constraints and some practical constraints.

“It’s likely that there will be some practical limits on what can be achieved compared with today,” he told the Commons Home Affairs Select Committee.

Asked if Britain would be as “safe as now”, he replied: “However good you are at this threat – and everybody recognises that the UK and its agencies in this field are very good – you are more effective if you are working with your partners.”

On terrorism, cyber-crime and human trafficking, Britain and the EU had a “shared analysis of the threat and a shared interest in fighting it collectively”.

“But just because something is a shared objective doesn’t mean it’s easy to achieve,” Sir Julian added.

The former UK ambassador to France was asked to give evidence ahead of the Prime Minister triggering Brexit – which will mean leaving Europol when withdrawal is completed.

Security experts have warned that maintaining access to EU law enforcement databases and data-sharing platforms is “integral” to day-to-day policing in this country.

Information that could currently be sourced in seconds or hours could take days or weeks to retrieve – posing a risk to the safety of the public.

Sir Julian also pointed to the benefits of the European Arrest Warrant (EAW), with 8,000 suspected criminals extradited from Britain – and a further 1,000 brought back to this country.

It has been suggested that the UK could seek to negotiate a separate extradition agreement with the whole of the EU, as Norway and Iceland have attempted to.

But Sir Julian said that would be “more complicated” than the EAW, particularly if it allowed some extradition decisions to be taken at a “political level”.

And, asked if such an agreement was possible, he added: “You are asking me to make predictions about a negotiating process that hasn’t even started yet. I don’t know.”

Sir Julian disagreed with Sir John Major, who warned this week of a “sour atmosphere”, but added: “The negotiations have not started yet.”

In November, the Home Office said it intended to remain a full member of Europol until Brexit is completed, giving the Government two years to agree on a new framework for co-operation.