By Peter Griffiths
LONDON (Reuters) - Britain's opposition Labour leader Ed Miliband suffered a double blow on Wednesday when one of his party's biggest donors slashed its funding and an opinion poll exposed serious doubts over his authority.
The survey, carried out early this week, suggested Miliband's standing had failed to benefit from the coalition government's historic defeat in the British parliament over military action in Syria.
Miliband, whose opposition was crucial in Prime Minister David Cameron's shock defeat last week, is already under pressure from some in his centre-left party to improve his lackluster personal ratings and mount a stronger challenge before the next election, due in 2015.
The latest survey gave more ammunition to Miliband's critics, suggesting voters see Cameron as stronger, more charismatic and more decisive. Just 2 percent thought Miliband is a "natural leader", compared to 13 percent for Cameron.
The YouGov poll for the Sun newspaper gave Labour a four point lead over Cameron's Conservatives, well down from the 10 point advantage a separate poll on Monday gave Labour after last week's vote. Twenty YouGov polls in August put the Labour lead at between three and eight points.
Labour lost power after 13 years in 2010 but Cameron failed to win an outright majority and was forced to enter a coalition with the Liberal Democrats.
In a setback for Labour three weeks before its autumn conference, the GMB union said it would cut donations to party campaigns and pass on less money from members.
The loss of an important source of income is the latest episode in a row between the unions and Labour over Miliband's plan to reform ties with the workers' groups that helped found the party more than a century ago.
Miliband, who relied on union support to defeat his brother David in his party's 2010 leadership contest, angered unions in July when he said Labour must transform its links with the labor movement to make politics more transparent.
The union said its decision reflected "considerable regret... that the party that had been formed to represent the interest of working people in this country intends to end collective engagement of trade unions in the party".
The changes would see the GMB's annual fees to Labour sharply cut to 150,000 pounds ($233,000) from 1.2 million pounds. The union did not say how much it would cut from its campaign donations.
Labour finance spokeswoman Rachel Reeves played down the cuts, saying it would raise more from individual members.
"Ed has spoken about the need for greater transparency and openness in that relationship," she told BBC radio.
Labour's planned union reforms are part of a wider debate over party funding and whether donations should be capped.
Cameron accuses Miliband of being in the unions' pocket, while his counterpart says the Conservatives are in the thrall of wealthy business leaders.
(Editing by Mark Trevelyan)