Britain and other non-eurozone nations must collaborate to ensure their national interests are not sidelined amid changes at the European Union provoked by the crisis in the single currency, Prime Minister David Cameron has urged. The Prime Minister said he was working to ensure safeguards were put in place to protect all 27 member states should, as the UK wants, the 17 euro countries introduce tighter fiscal integration. Conservative eurosceptics have raised fears that those in the currency bloc will "gang up" against the others.Cameron, who discussed the issue with fellow non-euro leaders in meetings last Monday while their counterparts wrangled over the details of the new package aimed at resolving the debt package, told reporters that there was clear recognition of the potential problem."As the 27 we need to make sure that the single market is adequately looked after," he told reporters travelling with him from the EU summit in Brussels to a meeting of Commonwealth leaders in Australia. "There are a lot of things the eurozone is doing together. Having more meetings alone, establishing machinery - it raises the question of could there be caucusing? (shut us out) "There is common agreement that this was an important issue and that we should be working together to make sure the arrangements provide a safeguard." And in a clear message to the European Commission that it had a duty to ensure fair operations across the board, he added: "It is very important that the institutions of the 27 are properly looked after and that the commission does its job as the guardian of the 27." He highlighted the City of London (the financial and banking district) as an area that faced increasing pressure and required protection. "London is the centre of financial services in Europe. It's under constant attack through Brussels directives. It's an area of concern, it's a key national interest that we need to defend," he said.