The Czech Constitutional Court is to begin considering what could be the last legal challenge to the ratification of the EU's Lisbon Treaty. <br/> <br/>The court will hear a complaint by 17 senators, who say the reform treaty infringes upon Czech sovereignty. They are backed by Czech President Vaclav Klaus – the only EU leader who has so far refused to sign the treaty. <br/> <br/>The treaty – which aims to streamline EU institutions – cannot take effect unless all 27 member states back it. It is under threat, not only by President Klaus, but also by the UK's Conservative Party. <br/> <br/>They say they will hold a referendum on the treaty if they form a government next year, and if it is not signed into law by then. The Czech parliament has already approved the treaty, but Mr Klaus is yet to sign it. <br/>