European Union leaders signed a landmark treaty designed to improve the way their 50-year-old organization functions during a ceremony in Lisbon on Thursday, according to dpa. The 250-page text was signed by all of the EU's 27 prime ministers or heads of state and foreign ministers except for Gordon Brown. The British premier had to delay his arrival in the Portuguese capital due to "long-standing diary commitments" in London but was expected to add his signature to the text in the afternoon. Officials from Belgium to the United Kingdom took turns to ink the treaty in the main hall of Lisbon's magnificent Jeronimos Monastery and were treated to the melodious music of Dulce Pontes, one of Portugal's most renowned singers. They then gathered for a group photo before boarding electric trams that were to take them to a festive lunch at the National Coach Museum. The Lisbon Treaty, formerly known as the Reform Treaty, seeks to simplify and speed up the EU's decision-making process and give the EU more clout on the international stage. It replaces a Constitutional Treaty ditched by French and Dutch voters in referenda held in 2005 and is the first major overhaul of EU rules since the Treaty of Nice was agreed on, almost exactly seven years ago. Under the Lisbon Treaty, a new EU president, appointed for up to five years, is to replace the current rotation system, in which member states take turns to hold the EU presidency for six months. It also updates the EU's executive body, the Commission, which draws up EU-level laws and makes sure that they are implemented. At present, every single EU member state nominates a commissioner, but the treaty reduces that number to 18. "The Treaty of Lisbon will reinforce the Union's capacity to act and the ability to achieve those goals in an effective way. As such, it will help the Union to deliver better results to European citizens," said EU Commission President Jose Manuel Barroso in a speech at the start of the ceremony. The treaty, agreed after months of wrangling among EU leaders, is designed to take into account the fact that the 50-year-old bloc has expanded to include 27 countries and a population of nearly 500 million people. All but two of the new member states that have joined the EU since 2004 were once on the eastern side of the Iron Curtain. "It is the treaty of an enlarged Europe from the Mediterranean to the Baltic, from the Atlantic Ocean to the Black Sea. A Europe that shares common values and common ambitions," Barroso said. "For the first time, the countries that were once divided by a totalitarian curtain, are now united in support of a common treaty that they had themselves negotiated," Barroso said. EU leaders were greeted under the Portuguese sun outside Lisbon's Jeronimos Monastery by Portuguese Prime Minister Jose Socrates and his foreign minister, Luis Amado. "History will remember this day as a day when new paths of hope were opened to the European ideal," said Socrates, whose country, along with Germany, played a fundamental role in getting ministers to agree on the text during the course of the year. The officials then put their signatures to the text inside the monastery, an imposing 16th-century building that houses the tomb of explorer Vasco da Gama and other prominent figures of Portuguese history. Britain was represented at the ceremony by Foreign Secretary David Miliband, who arrived on time in Lisbon despite his plane being delayed. The treaty must now be ratified by all 27 member states before it can come into force. EU leaders are to meet again in Brussels on Friday for a traditional end-of-year summit.