Brussels has expressed its reservations about the restructuring package put forward by the government to save troubled mortgage lender Northern Rock, including fears that state aid is likely to drag on for too long.

The mortgage lender adopted a business plan allowing it to pay off a £26bn Bank of England loan within three years. It was also ordered to halve its mortgage book by encouraging borrowers to switch to other lenders.

The bank has a three-year guarantee that any savings deposited with it will be backed by the government.

The European Commission’s first assessment of the plan, set out in the European Unions official journal, sys it “doubts whether the size and duration of this down-sizing are sufficient to avoid undue distortions…[or] that… a deeper and more rapid downsizing could not be implemented”.

The regulator has particular concerns that the restructuring plan may run for an unnecessarily long period, that the state aid could have been more limited and that more could be done to compensate rivals over distortions to competition.

Lawyers have said that the commission could order the aid to be repaid if it was found to have been given illegally, but the decision on the rescue would be made in the second half of this year.

The chancellor Alistair Darling is said to be “pretty relaxed” about the commission’s assessment of the rescue plan. How relaxed he will remain is debatable as the EU journal does set out significant concerns about its compatibility with EU state aid rules.

Mr Darling’s officials were quick to maintain that the assessment came as no surprise and was simply part of a standard Brussels procedure when launching a state aid enquiry.

The proposed rescue operation was discussed at every stage with Neelie Kroes, the competition commissioner, and her officials before it was put into action insist the Treasury. Northern Rock was taken into state ownership in February.

Publication in the Official List formally triggers a month-long period in which business rivals and other interested parties can submit comments on the restructuring deal to the commission, although some views may have already gone in.