European Union Commission President Jose Manuel Barroso says he will field new proposals by the end of the month to tackle some of the actions of market speculators, who many have blamed for making the financial crisis worse.

The 1948 nude, entitled “Olympia” and estimated to be worth up to £3.65 million, was taken from the René Magritte Museum, located in the house where the Belgian surrealist artist lived for 24 years. During the robbery, which took place...