(DUBLIN) - Most Irish people do not understand the European Union's key Lisbon Reform Treaty which they will vote on in a referendum on June 12, an independent commission said on Monday.

A survey carried out for the Referendum Commission this month found that 80 percent said they did not understand the treaty particularly well, or at all.

Just five percent said they understood the treaty, while 15 percent said they understood it to some extent.

The Referendum Commission has the job of explaining the treaty to voters, promoting awareness that the referendum will take place and persuading people to vote.

Ireland is the only one of the 27 EU nations planning to hold a referendum on the treaty. A "No" vote could scupper it altogether.

In 2001, Ireland shocked the EU when it rejected the bloc's previous Nice Treaty on institutional reform and enlargement. That decision was later reversed in another referendum.

The Lisbon Treaty -- replacing the bloc's doomed constitution rejected by French and Dutch voters in 2005 -- was agreed last December amid much fanfare.

It aims to prevent decision-making gridlock in the expanding bloc.

A newspaper poll Sunday showed a substantial shift in public opinion against the treaty, with the number planning to vote "Yes" dropping to 35 percent, down eight percentage points on a similar poll two months ago.

Some 31 percent of Irish citizens said they would vote "No", an increase of seven points, in the Sunday Business Port/Red C poll.

The number of undecideds had increased by one point to 34 percent.

The swing against the treaty is particularly marked among farmers. They claim the EU stance in talks over a world trade deal would damage Irish agriculture.

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