"In 2007, three large ('L-class') missions, namely, EJSM-Laplace, IXO and LISA, were selected for an Assessment Phase study following a Call for Missions for the first L-class mission opportunity in ESA's "Cosmic Vision 2015-2025" plan. These Assessment Phase activities have recently been completed. The launch of the first L-class mission ('L1') was planned to take place in the early 2020's.

All three L1 mission candidates have been proposed to ESA, and have been studied, as cooperative enterprises with international partners, with NASA in a key role in all three. As a consequence, decisions on the future of these missions cannot be taken in isolation from the prospective partners.
Recently, the US National Research Council (NRC) has released 'decadal' surveys for both astronomy and planetary science. While both surveys recommended continued cooperation with ESA, and ranked highly all three L1 mission concepts, none of them were ranked as top priority. The US budget outlook also became known in February 2011, and in ESA's recent discussions with NASA it became clear that it is quite unlikely that any of the L-class mission candidates can be implemented as a joint Europe-US mission in the planned timeframe of the early 2020's. [...]

New, European-led science teams have been formed, and, with the support of ESA's engineering teams, they have been asked to examine if, and to what extent, the science case of each of the three original L1 mission concepts could be implemented in the context of affordable European-led missions with possible limited international participation to be launched in the early 2020's. ESA is jointly assessing with its international partners if, and at what level, international participation would be possible on these new mission concepts.
As is customary for programmatic decisions, ESA will request the Space Science Advisory Committee to review and assess the science case of each of the three new mission concepts resulting from this activity, prior to a recommendation on the new mission concepts being proposed to the Science Programme Committee in February 2012." http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=48661