Speaking in Parliament on Tuesday, Sionaidh Douglas-Scott told MEPs, "The clock is ticking and I find this unpreparedness a very worrying prospect. The question is whether it is all going to be possible and there is a lot for the UK to be worried about."
Her comments before the committee on constitutional affairs came a day after formal Brexit talks got underway in Brussels.
Douglas-Scott, of the University of London, told a special hearing on Brexit that the UK faces a huge task translating EU legislation into UK law.
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"The UK government says it has 100 pages of details on its Brexit negotiating position, which sounds impressive until you realise that this amounts to no more than a couple of white papers that it has produced. So far, we have not heard any more than that and there has been very little in the way of detail from the UK side," added Douglas-Scott, a constitutional and EU law specialist.
The hearing was convened to debate the constitutional aspects of Britain's decision to leave the EU.
The academic, also a member of the UK Constitutional Law Association, said the decision had thrown up unprecedented legal, constitutional and regulatory complexities which had to be thrashed out before Brexit talks are due to be concluded in less than two years' time.
"The UK is going to lose a huge amount of legislation when it withdraws but I see very little evidence that the UK is prepared for all this, particularly in view of the recent unnecessary general election, losses of key members to the UK Brexit team and cuts in the UK civil service," she told the meeting.
The EU referendum result would probably mean a special agreement having to be found for Northern Ireland which, she reminded the audience, had voted by 56 per cent to stay in the EU.
The result of the recent British election, which resulted in the Tories having to rely on the DUP from Northern Ireland to shore up its tiny parliamentary majority, had thrown another spanner into the works, she noted.
"No one wants a return of a hard border in Ireland but the question is this issue will be resolved. It may be that Northern Ireland will still be able to retain single market or customs union membership."
But any concessions offered to Northern Ireland, she said, could well result in Scotland, which voted 62 per cent to remain in the EU, making demands of its own.
Douglas-Scott, who is a special adviser on EU affairs to the Scottish Parliament, said, "Scotland could rightly claim that, if it is possible for o Northern Ireland to have a special agreement then why not Scotland as well."
She added, "This whole Brexit business reflects the UK's relationship with the EU but it also tells us a lot about the UK's constitutional law and our flexible constitution."