Conservative minister Ken Clarke has joined forces with Liberal Democrat Danny Alexander and Labour's Lord Mandelson to warn that leaving the EU would be an "historical error".
The politicians insisted the UK should work to reform the organisation from within rather than walking away.
Mr Clarke, regarded as the sole Conservative europhile in the Cabinet, said: "David Cameron has already shown what can be achieved in Europe through strong, determined leadership.
"We earn our living in a globalised economy and a world in which nations are interdependent in tackling global and regional problems.
"We can help to promote our own interests best by reinforcing our role as a leading nation in the EU."
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The Prime Minister said Parliament will be given a binding vote on the final deal, but said the result will not stop Britain leaving the EU.
Officials from the train drivers' union Aslef will meet managers from Southern Railway, whilst the RMT will meet separately with Acas.
Last week the organisation wrote on Twitter: "If Obama grants Manning clemency Assange will agree to US extradition".