British tax havens would have just six months to open their books to mainland scrutiny or face being placed on an international blacklist, if Labour wins the general election, Ed Miliband has declared.
Opposition leader Mr Miliband also accused David Cameron of failing to follow through on demands that all overseas territories and crown dependencies adopt transparency measures being introduced in the UK.
Mr Miliband said: "The current Conservative leadership have become the political wing of offshore hedge funds.
"Unlike them, we will not stand by. We will ensure a country where everyone plays by the rules, from top to bottom."
Agreement on action to expose the owners of "shell companies" used to evade tax was hailed by Mr Cameron as a key achievement of the UK-hosted G8 summit of world leaders in 2013.
But so far none of the countries around the world over which Britain retains sovereignty has accepted the Prime Minister's appeal to them to "move forward together in raising standards of transparency" and some have ruled out reform.
Mr Miliband, who has endured a torrid few days of attacks by business leaders over the potential impact of his party's policies accused David Cameron of 'dithering' over the issue.
In a letter to the leaders of the overseas territories and crown dependencies, Mr Miliband put them "on notice" that, if elected in May, his government would refer any that failed to produce publicly accessible central registers of beneficial ownership - who profits from a company - to the Organisation for Economic Co-operation and Development (OECD).
"I am writing to put you on notice that a Labour government will not allow this situation of delay and secrecy to continue," he wrote.
"Labour will act on tax avoidance where the Tories will not," he added - ending protection from international scrutiny and requesting OECD blacklisting."