British Prime Minister David Cameron holds his own in TV debate

Prime Minister David Cameron won the first TV encounter of a close national election in Britain, an opinion poll showed, but opposition Labour leader Ed Miliband leveraged a rare opportunity to promote himself on a national stage.

With Cameron's Conservatives neck-and-neck in the opinion polls with Labour, both are trying to grab an elusive lead over the other before the May 7 ballot to avoid another coalition or a minority government.

Britain's future in the European Union and its territorial integrity could ride on the outcome as Cameron is pledging an EU membership referendum and Miliband may need to do a deal with Scottish nationalists intent on breaking up the UK.

Cameron and Miliband were interviewed separately but back-to-back in their toughest cross-examinations in years, and subjected to question-and-answer sessions from a TV studio audience on Sky News and Channel 4.

A snap Guardian-ICM poll afterwards showed 54 per cent of those asked thought Cameron, the leader of the Conservative Party, had won, compared to 46 per cent who judged Miliband had triumphed.