Copenhagen- Danish voter turnout on Sunday for elections to the European Parliament will prove decisive for a planned change of the law on the order of succession to the throne, as voters will be asked to cast their ballot in an accompanying referendum.

The proposed change - earlier approved by parliament - was to ensure that the first-born child of the heir to the throne was to be first in line, regardless of gender.

For the change to be approved, a majority of voters have to vote for the bill and they must constitute at least 40 per cent of the some 4 million registered voters.

Some recent surveys suggested the threshold would not be achieved. Polling stations close 8 pm (1800 GMT).

The change will apply in future since Crown Prince Frederik's oldest child, Prince Christian, was born before his sister, Isabella. Christian - born October 15, 2005 - is second in line to the Danish throne after his father.

The current law of succession was introduced 1953. Queen Margrethe, who became monarch in 1972, is the oldest of three sisters.