Even if voters do not give Erdogan the executive presidency he seeks on Sunday, Turkey's democracy and judiciary will suffer damage and he is likely to put even more pressure on critics, they say.
Marc Pierini, a former European Union ambassador to Turkey now at the Carnegie Europe think-tank, said: "There is no good outcome.
"There is a very wide gap between European leaders and Erdogan and I don't see that easily repaired."
President Erdogan's calls for sweeping new powers is unlikely to ease strained relations with the EU
Turkey, a NATO member state that began talks to join the EU in 2005, has become a crucial partner for the bloc by taking in millions of refugees fleeing from the six-year-old war in Syria.
But a crackdown by Erdogan since a failed coup last July has been condemned in European capitals, and he has alienated the bloc further by accusing the German and Dutch governments of acting like Nazis after they banned referendum campaign rallies by Turkish officials.
Turkey is a NATO member state that began talks to join the EU in 2005
One senior official said an Erdogan victory, which would pave the way for the president being eligible to serve up to two five-year terms, could bring stability allowing the EU to upgrade trade and business ties. "Otherwise it would be chaos," he said.
There is no good outcome
But several others disagreed, with a second senior official rejecting that notion as "false stability in the name of one-man rule".
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With Erdogan supporters seeing a chance to cement his place as modern Turkey's most important leader while his opponents fear an even greater centralisation of powers, polls suggest the vote – in which Turkish citizens abroad are also participating – will be close.
EU officials see all outcomes – whether a clear victory for Erdogan, a narrow loss or a contested result – as risky.
A win would embolden Erdogan to move ahead with proposed constitutional changes
A win would embolden Erdogan to move ahead with proposed constitutional changes and possibly introduce the death penalty, they say, thereby ending Turkey's bid to join the EU.
Its status is already an issue following the post-coup crackdown.
Legal experts at the Council of Europe, a human rights body of which Ankara is a member, warned in March that creating a presidency with virtually unlimited powers was "a dangerous step backwards" for democracy.
Erdogan and his supporters dismiss such claims, saying there are sufficient checks and balances in the proposed system, such as the president having to call simultaneous presidential and parliamentary elections if he dissolves parliament.
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Other scenarios are also bleak, EU officials say.
A contested result would likely provoke a period of instability, possibly more violence.
A narrow loss would see Erdogan, whose position as Turkey's most popular politician is uncontested, remain in office.
EU officials said he could bring the presidential and parliamentary elections, currently scheduled for 2019, forward both if he loses and if he wins.
A third senior EU official said: "If he loses, we can expect a very harsh and brutal election campaign, we would probably see both sides resorting to violence.
"He would crack down on the opposition even more (and) … if the result is contested, it could destabilise the overall situation."
Erdogan's crackdown after a failed coup last July has been condemned in European capitals
Following the failed coup, 36,000 people are in Turkey's prisons awaiting trial and more than 100,000 have been suspended or dismissed from work, according to the Council of Europe.
The ideal scenario from Brussels envisages a period of post-referendum calm that would allow the EU and Turkey to reset relations and modernise their customs union, possibly even relaxing visa rules for Turks travelling to the bloc.
A prerequisite for that is a breakthrough in Cyprus reunification talks, where Ankara is facing off against Nicosia and Athens, the third official said.
He said, anticipating Erdogan will move to review ties with the EU whatever happens: "There is no entirely good result of this for us in any case. But some scenarios would be worse than others."