The UK will be ready for crashing out of the EU without a deal, but “not everything will be perfect” and there could be “horrendous” consequences if co-operation breaks down completely, a Whitehall mandarin told MPs.

Theresa May has ordered officials to step up planning for a “no deal” Brexit as part of the Chequers plan.

Whitehall’s most senior officials insisted the UK would be able to cope if the UK’s exit happened on March 29 next year without a formal deal with Brussels, but acknowledged there would be disruption for both Britain and the EU.

Civil service chief executive John Manzoni told MPs: “We will be ready for a no deal Brexit. Not everything will be perfect.”

Acting Cabinet Secretary Sir Mark Sedwill added: “That puts it very well – we will be ready but we shouldn’t assume it will be smooth, if it’s a disruptive outcome.”

Mr Manzoni said the UK was doing what it could but it was difficult to determine how “third parties” such as EU member states, their local governments or private companies would act – and whether they would be “spiteful or ignorant” about the consequences.

There were “some things that really are very complicated in the absence of co-operation” such as data sharing, which is currently covered by EU agreements.

“These things are all in everybody’s interests to get right but, on the other hand, we have to prepare in the event that there are either spiteful or ignorant or whatever activities by third parties,” Mr Manzoni said.

That would make it “very uncomfortable” and could have “some horrendous consequences” but “that’s what we have got to try and do our best to mitigate against”.

Acting Cabinet Secretary Sir Mark Sedwill told MPs: “We wouldn’t want any of our European partners to think that we aren’t well prepared enough that it becomes a negotiating card on their side of the table.”

He added: “We do have confidence in our ‘no deal’ planning, we know that it will be disruptive for them and for us if there is no deal and that we haven’t been able to find some other arrangement.”

Sir Mark, who is also the National Security Adviser, indicated that plans developed for other crises could be put in place.

“We have a whole range of mechanisms, some of them designed over many years for different circumstances that we can apply should that be the case against a range of no deal scenarios.”

Giving evidence to the Public Administration and Constitutional Affairs Committee, Mr Manzoni said 5,800 extra civil servants had been recruited to cope with Brexit, with 1,000 more in the pipeline.

Mr Manzoni acknowledged more work was needed before March 2019: “I’m not sitting here today saying it’s all going swimmingly well and we are ready, because we are not.”

Detailing the “no deal” plans Mr Manzoni said: “There are supply chains for food and medicines and such things, we have to put in place contingencies for those.”

The Department of Health has announced it will stockpile medicines, he said.

There were also “transport systems” that would have to be dealt with, particularly for the aviation sector, and measures would have to be put in place to take care of UK nationals overseas.