Dr Eurfyl ap Gwilym said 70% of the budget was on health and education so cuts were inevitable
Job cuts and a future end to free prescriptions in Wales look inevitable in the current climate, Plaid Cymru's top economic adviser has told the BBC.
It comes as the Welsh budget faces a £600m a year squeeze from 2011 to 2014.
Plaid leader and minister Ieuan Wyn Jones warned of "big cuts" but strongly defended free prescriptions.
The current economic climate is also likely to see the £1bn M4 relief road project around Newport, south Wales, put on hold, BBC Wales has learnt.
Industry sources have indicated to the BBC Wales programme that financially the M4 relief road project no longer stacks up because of a combination of soaring costs of up to a billion pounds and a lack of public or private capital.
John Griffiths, Labour AM for Newport East, and a deputy minister in the assembly government said in the current circumstances he found it difficult to imagine this amount of money being found.
It was very hard to see the scheme being given the go ahead in the near future, he added.
BBC Wales Dragon's Eye programme has also established that the assembly government has spent nearly £20m on preparatory work for the project to date.
Transport experts have told the programme that the last official costing for the scheme - around £350m - is now hopelessly out of date, and that a more accurate figure is between £800m and £1bn.
In September 2007, the deputy first minister said the road would run from Magor to Castleton and could open in 2013.
His comments followed two M4 crashes which led to traffic chaos.
Since then, further crashes followed by long delays have prompted business leaders to call for the road to be built.
However opponents of the proposed road have claimed it would wreak havoc with wildlife along its proposed route.
A new relief road is now estimated at costing between £800m and £1bn
But apart from big capital projects, Dr Eurfyl ap Gwilym, Plaid's top economic adviser, said there would have to be job losses beyond natural wastage to make ends meet.
Even areas like health and education would have to carry some of the cuts, he said.
"It's unfortunate, but the reality is that the finances of the UK are in a total mess," he said.
"At the end of the day if you look at the WAG [Welsh Assembly Government] budget the overwhelming majority of it - about 70% - is focused on health and education and therefore if you're going to make reductions those two places will unfortunately have to carry some of the cuts. Hopefully lower than in other areas.
"We've got to look at a number of areas. Clearly one would be free prescriptions for those on higher incomes, that's one.
"We are now going to bring in top up university tuition fees - I think that's an unpleasant decision but one that needs to be taken.
"I do think we need to look very hard at where money's been spent on economic development.
"Wales has spent very large sums in the last nine or 10 years and the outcome has been pretty disappointing.
"Again, not saying just arbitrary cuts but we need to look very hard at where the money is going and talk to the business community to see where best to spend that money."
Ieuan Wyn Jones, deputy first minister and Plaid leader, said it was still unclear what the real figures after 2011 would be, but added: "What we do know, and there's no dispute about it, is because of the state of the public finances, the Welsh block grant will be severely affected, not just next year, but well, well into the next decade."
"Let me make it perfectly clear, these are big cuts, there's no question about that. And if the Conservatives were to win the next election, then I anticipate that the cuts will be even larger. So there is no doubt, going forward into the next decade, Welsh public services will be badly affected."
But he strongly defended the free prescriptions scheme, which he believed was necessary for people.
"But there may well be other things that you have to look at," he said.
"I'm not going to write the next election manifesto for the party here on Dragon's Eye, but what I will say to the people of Wales is - we will be sensible in the way we approach it."
Kirsty Williams, leader of the Welsh Liberal Democrats, said Wales needed to be prepared for cuts and called for First Minister Rhodri Morgan to reveal where these would be made.
"At a time of economic uncertainty, the last thing people want is an indecisive government. Plaid Cymru, along with their Labour coalition partners, need to learn to cut their cloth accordingly," she said.
Welsh conservatives leader Nick Bourne AM said his party had repeatedly told the assembly government that efficiency savings alone would not be enough to deliver the cuts.
"We have also warned Labour and Plaid Cymru that they need to re-focus spending on frontline public services and not the sort of expensive gimmicks and freebies which have been the hallmark of this administration," he said.
Dragon's Eye will be broadcast on BBC One Wales at 2245 BST on Thursday.