By Oana Lungescu
BBC correspondent in Brussels
The European Commission has signalled that it may suspend membership negotiations with candidate countries if they fail to meet tough EU criteria.
Guenter Verheugen steered the EU's enlargement this year
The new conditions are set out in an EU strategy document as the Commission prepares to recommend the start of membership talks with Turkey.
The keenly awaited reports on Turkey, Bulgaria and Romania will be released on Wednesday.
Any backtracking on human rights or democracy would delay the negotiations.
The man who will oversee any future negotiations with Turkey, incoming enlargement commissioner Olli Rehn, also made it clear he wanted a permanent mechanism that would prevent large inflows of Turkish workers into the EU.
The top aides to the 25 European commissioners have put the finishing touches to the reports that will be unveiled on Wednesday.
Officials say they support the conclusion that Turkey has made sufficient progress to begin membership talks.
But at the same time, the EU will signal a major shift in how it deals with prospective members.
Starting with Croatia next year, membership negotiations will be longer and harder. Promises of reform will no longer be enough to conclude negotiations.
According to the strategy paper - seen by the BBC - benchmarks for the implementation of reforms will have to be met.
And for the first time, the EU will introduce a break clause - the possibility to suspend negotiations in the case of a serious and persistent breach of the principles of liberty, democracy, respect for human rights and fundamental freedoms.
Mr Rehn, who will take over as European enlargement commissioner on 1 November, told the European Parliament he would also insist on a permanent safeguard clause for Turkey.
It will allow the EU to close its borders to large numbers of labour migrants at any point in the future - not just for seven years, as is the case for Poland and the other former communist countries that joined the EU in May.
This has never been done before and it may fuel Turkey's suspicion that it is treated like no other candidate.
But Mr Rehn said it was needed to calm public concerns. This, he concluded, will not be an easy decision - it is a question that divides European public opinion.
Bulgaria and Romania, which are far advanced on the road to membership, will be affected to a certain degree, but not as much as the other outstanding candidates.
The strategy paper concludes that while Bulgaria and Romania continue to fulfil the political criteria for EU membership, "improvements need to be made in particular in the reform of their public administration, the functioning of their judicial system and the fight against corruption".
The paper says that both Bulgaria and Romania fulfil the criterion of being a functioning market economy and the "continuation of Bulgaria's reform path and the vigorous implementation of Romania's structural reform programme should enable them to cope with competitive pressure and market forces within the Union."
Both countries also need to develop "sufficient administrative and judicial capacity to implement and enforce" EU laws.
The European Commission says the objective is to sign a joint accession treaty with both countries as early as possible in 2005, which should enter into force on 1 January, 2007.
But Mr Rehn warned that, under the new safeguard clause, if either of the two countries failed to come up to European standards, their entry could be delayed by one year to 2008.
"This clause is not included in the accession treaty just for fun," Mr Rehn said. "It's a serious clause and we would not hesitate to use it."