Spanish singer Massiel's La, La, La pipped Sir Cliff's hit song Congratulations by one vote at the last minute in the contest which was held at London's Royal Albert Hall in April 1968.

Now 40 years on, filmmaker Montse Fernandez Vila has revealed in a documentary, titled 1968. I lived through the Spanish May, that Spain's only ever Eurovision win was down to behind-the-scenes negotiations by television executives from that country's state-run channel.

The filmmaker has claimed that the executives toured Europe offering cash and promising to buy TV series and enter into contract with unknown artists from other Eurovision member states to influence the vote in the singing contest.

"There is evidence that votes were bought to secure a win for Massiel. The (Franco) regime was acutely aware of the need to improve their image (both at home and abroad).

"Looking back at the parties that were organised and the way Massiel was turned into a national hero – it seems a bit excessive for a song festival but it all served to glorify the regime," The Daily Telegraph quoted Villa as discussing the documentary with Spanish newspaper 20 Minutos.

Massiel, now 60, whose real name is Maria Felix de los Angeles Santamaria Espinosa, went on to become one of Spain's best loved singers and re-released her Eurovision entry last year with a hip-hop beat.