The Queen and the Duke of Edinburgh are to visit Turkey for the first time since 1971, Buckingham Palace announced today.

The State visit in May, at the invitation of President Abdullah Gul, comes in the midst of a protracted struggle over Turkey’s efforts to join the European Union.

The country has been knocking at the EU’s door for more than 40 years, but the country only became an official candidate in 1999. Formal accession negotiations began in October 2005, with Turkish EU membership seen increasingly as a strategic bridge to the Middle-East.

Turkey would be the first secular nation with a majority Muslim population in the EU club, but Ankara has received repeated warnings about the scale of social, economic and human rights reforms required before the country can join.

Since formal EU talks began, the death penalty has been abolished, tougher measures introduced against torture, and the justice system partially overhauled.

But attempts to prosecute leading writers for a criminal offence of insulting “Turkishness” have soured negotiations with Brussels. Franco Frattini, the EU Justice Commissioner, warned Turkey recently that it was “far from EU accession”.

“Turkey has many deficiencies in terms of freedom of speech, women’s rights and prison standards,” he said, adding that the EU could not talk about Turkish membership before such obstacles were overcome.

At the end of 2006 membership negotiations faltered, getting back on track a year ago after having been partially frozen for three months in a dispute over Turkey’s reluctance to recognise Cyprus, still divided since Turkey seized the northern third of the island in 1974.

The Queen and Duke last visited Turkey in October 1971, when membership of the then Common Market was a distant aspiration. Even today, completing the membership process could take another decade, according to some EU negotiators.

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