By Paul Taylor

BRUSSELS (Reuters) - Former European Commission president Jacques Delors, the father of modern European integration who became a bogeyman for British eurosceptics, said on Thursday that Britain's EU membership was positive both for the United Kingdom and for the European Union.

Delors, 90, who clashed frequently with British Prime Minister Margaret Thatcher during his decade at the head of the EU executive from 1985 to 1995, said in a statement to Reuters that he would respect the British voters' decision in next Thursday's referendum on whether to remain in the EU.

"I consider the UK's participation in the European Union to be a positive element both for the British and for the Union," the French Socialist elder statesman said.

His statement was emailed through the Jacques Delors Institute to dispel what it called inaccurate rumors that he favored a Brexit so that remaining EU members can move ahead with deeper integration.

It came as several opinion polls showed that supporters of Britain leaving the EU had taken a lead in the hard-fought campaign for the June 23 vote, due to fears of immigration from Europe.

In a separate article released on Thursday, Delors joined former EU commissioners Pascal Lamy and Antonio Vitorino, former Italian premier Enrico Letta and the Delors Institute's director, Yves Bertoncini, in calling for a stronger European collective security effort, whatever the outcome of the British vote.

"Every country in Europe should contribute to strengthening our collective security, and that includes the United Kingdom, which will participate even better as a full member of the EU," they wrote. Britain and France are the EU's leading military powers with the most extensive intelligence services.

"Where security and numerous other global challenges are concerned ... (Prime Minister) David Cameron is absolutely right to highlight the fact that we are 'stronger together'."

"UP YOURS, DELORS!"

It was not clear what impact, if any, Delors' endorsement of UK membership would have on the inwardly focused British debate.

Many Britons recall that Thatcher's angry rejection of his vision of a federal Europe, with the Commission as an effective continental government in Brussels, led to her overthrow by rebels in her Conservative Party in 1990, opening an internal feud that has endured for more than quarter of a century.

The eurosceptic Sun tabloid newspaper famously splashed the headline "Up Yours, Delors!" across its front page, and a young journalist called Boris Johnson, now a leader of the Leave campaign, was Delors' tormentor in the Brussels press room.

Fewer people are aware of how closely Delors and Thatcher cooperated to promote the integration of the European single market in the 1980s via a treaty reform that greatly extended decision-making by majority vote among member states.

Delors was also the main architect of the euro single currency, which Britain has never joined.

A small "good riddance" camp of supporters of a federal Europe, notably former French prime minister Michel Rocard, have openly expressed a desire to see Britain leave.

But most serving politicians and EU officials see the UK as a valuable, if semi-detached, member of the 28-nation Union and fear a decision to withdraw would harm Europe's global standing and might lead to a broader unraveling.

Delors, who is in weakening health, has largely kept out of public debate about EU and French national affairs in the last few years but a book based on his archives and conversations with journalist Cecile Amar was published this year in France.

(Editing by Alastair Macdonald and Robin Pomeroy)