LORD O’Donnell, the former Cabinet Secretary, has warned that it will take “at least five years” for the UK to negotiate a final Brexit deal with the European Union.

The former civil service chief also made clear that “some of the transitional arrangements may be longer than that”.

His remarks follow on from those of Sir Ivan Rogers, Britain’s ambassador to the EU, who, it emerged, told ministers in October that securing a trade deal with the Brussels bloc post Brexit could take as long as 10 years; a notion brushed aside by the UK Government and dismissed as “gloomy pessimism” by one leading Brexiter.

Read more: UK Government brushes aside suggestion post-Brexit trade deal could take 10 years to finalise

Lord O’Donnell, who served as Cabinet Secretary under Tony Blair, Gordon Brown and David Cameron, said: “We certainly won’t have come to any final arrangements in two years’ time. We might well get to a point where we can symbolically leave but all sorts of details will still remain to be sorted out.”

He told BBC Radio 4’s Westminster Hour: “We’ll have got some arrangement whereby we can say, right, from now on we’re no longer going to be governed by the European Court of Justice. But it will still be unclear precisely what the deal will be for all sorts of parts of goods and services for our trade, and certainly may well be unclear about what access we might have to their markets.”

Asked if he agreed with Chancellor Philip Hammond’s recent comments that “thoughtful politicians” and civil servants were coming to the view that a transitional arrangement would be necessary to smooth the UK’s exit from the EU, the crossbench peer said: “That’s a statement of the completely blindingly obvious.

“I mean the idea that you can manage this carving out of a new relationship between the UK and the EU in 18 months let alone two years, there’s not a chance, there never was a chance. That’s not to say we can’t have symbolically left. But it means that we’ve got to get our heads round the idea that leaving may be a symbolic act and that lots and lots of the details will still remain to be sorted out, so the uncertainty will not have gone away.”

Read more: UK Government brushes aside suggestion post-Brexit trade deal could take 10 years to finalise

Asked how long the process of reaching a final deal would take, Lord O’Donnell replied: “Years and years,” adding: “I can imagine it taking at least five years to get through all of the details. And I imagine some of the transitional arrangements may be longer than that.”

In October, Sir Ivan told ministers in October that other EU members believed a trade deal might not be hammered out until the early to mid-2020s and that EU leaders thought the final Brexit deal was likely to be a free trade arrangement rather than continued single market membership.

After former minister Dominic Raab, a leading Brexiter, dismissed the ambassador as a “gloomy pessimist,” Mark Garnier, the International Trade minister, told MPs it was "very, very difficult" to work out how long any trade deal would take, pointing to the swift four-month period needed to secure an agreement between America and Jordan and the longer Trans-Pacific Partnership[TPP], which had taken potentially eight years.

He described Sir Ivan’s remarks as "words from interlocutors" rather than a definition of how long it would take to create a UK-EU trade deal.

But Labour called on the Government to give investors the certainty they "desperately need" and outline the UK's hopes for trading arrangements with EU member states.

Bill Esterson, the Shadow International Trade minister, told Mr Garnier: "Today, we are told it could take up to 10 years to reach a trade agreement with the EU after we leave while research from NIESR[the economic think-tank] suggests a drop in trade of up to 60 per cent if we're outside the customs union.

"Foreign investors are vital to the British economy, so will you give those investors some of the certainty they so desperately need and we also need as well?

"Will you tell them whether you want Britain to be inside the customs union and whether you want tariff-free access to the single market or not?"

Read more: UK Government brushes aside suggestion post-Brexit trade deal could take 10 years to finalise

Mr Garnier replied by insisting the Government was not giving a running commentary on the UK strategy.

"I would also stress that the comments of Ivan Rogers are the opinions, taking words from interlocutors and this is not necessarily defining how long it'll take to create a trade deal.

"It's worth bearing in mind that if you like at some trade deals around the world, whilst TPP has taken potentially eight years, it's worth bearing in mind the US-Jordan trade deal took just four months.

"So it is very, very difficult to be able to establish exactly how long any trade deal will take," he added.

Chris Leslie, a former Shadow Chancellor, warned Britain faced a "mammoth bureaucratic task".

He told Liam Fox, the International Trade Secretary: "Shouldn't we be thanking our ambassador to the European Union for the reality check that he's given about the decade-long period of time it will take for us to extricate ourselves from this particular process? Shouldn't we be not rushing so headlong into this timetable?"

Dr Fox replied: "Yes, there are a number of bureaucratic challenges that we face but the people we should be thanking are the British people for giving us such clear instructions to leave the European Union."

He added: "We have repeatedly set out our worries about the slowdown in the growth of global trade. That has implications across the globe and it's worth making the general point that we need more free trade because it will increase global prosperity."

Meantime, the Secretary of State issued a blunt response to the SNP's Tasmina Ahmed-Sheikh, who said that while the Government's target was to double exports by 2020, the Office for Budget Responsibility expected UK trade to reduce as a result of leaving the EU.

Dr Fox replied: "I'm tempted to ask her if she would like Santa to bring a dictionary, because expectations and targets are not the same thing."