By Kylie MacLellan
LONDON (Reuters) - Britain's opposition Labour Party came under pressure to offer voters a membership referendum on leaving the European Union on Wednesday after a senior official quit over the issue and a major donor prepared to call for a policy U-turn.
Unlike Conservative Prime Minister David Cameron, Labour has resisted offering such a referendum in the run-up to a close May 7 national vote, arguing it would hang a "closed for business" sign over Britain and that an EU exit would be disastrous.
It has said it would only offer a referendum if it deemed there was a substantial further shift of powers from London to Brussels, something that's neither imminent nor likely.
But some in the left-wing party, parts of which have a tradition of Euroscepticism, argue it's undemocratic to deprive voters of a say on something polls show many feel uneasy about.
There's also disquiet -- from a tactical viewpoint less than three months before the election -- that Cameron's Conservatives are the only ones offering to renegotiate Britain's EU ties before holding such a referendum if re-elected.
On Wednesday, a former head of Labour's ruling body, the National Executive Committee, said she was leaving the party and switching her support to the anti-EU UK Independence Party (UKIP) because she was "disillusioned" by its Europe stance.
Writing in the Daily Telegraph newspaper, former NEC chair Harriet Yeo said she had been told many senior Labour lawmakers favoured a referendum but had been asked not to speak out.
"I cannot support this approach," she wrote. "It is time to decide whether we remain in the EU. The only party I trust to offer us that choice is UKIP."
Her resignation came as a major Labour donor prepared to urge Ed Miliband, the party's leader, to commit to hold a referendum.
"If Ed Miliband becomes prime minister in May and renegotiates without committing to a referendum, he will inevitably weaken the UK's bargaining position," businessman John Mills, who donated 1.65 million pounds to Labour in 2013, will tell a conference in London.
In remarks prepared for delivery to the event on alternatives to EU membership, he will say that other EU members will be more likely to take renegotiation seriously if there's a substantial risk of Britain leaving the EU.
