Campaigners stand during the unveiling of an election poster for Britain's opposition Labour Party in London, Britain May 11, 2017. REUTERS/Neil Hall

LONDON (Reuters) - Britain’s opposition Labour Party would crack down on tax avoidance and introduce a “Robin Hood Tax” on financial transactions to fund public services if it wins next month’s national election, it said on Saturday.

Labour, which is trailing Prime Minister Theresa May’s ruling Conservatives badly in opinion polls, said that 4.7 billion pounds ($6.1 billion) could be raised through a Robin Hood Tax by modernizing the tax regime on share trading.

It would close a loophole for banks and hedge funds and make other forms of financial assets, including financial derivatives, like options on shares, liable.

Labour would also take on those in Britain who dodge paying tax, closing loopholes and clamping down on tax havens.

“All we’re asking for is fairness in our tax system,” said Labour’s finance spokesman John McDonnell.

Last week Labour said middle and low earners would not face tax hikes to fund its spending plans if it wins power.

According to a leaked copy of its draft manifesto Labour will also promise to renationalize rail and mail services and take some of the energy sector into public hands, betting that a shift to the left will win over voters.

The document was mocked by Conservative supporters for echoing Labour’s left-wing 1983 election manifesto, described at the time by one Labour lawmaker as “the longest suicide note in history” for helping the Conservatives to victory.