Republican White House hopeful Mitt Romney is losing favour among US voters.

A new poll shows that 49% of voters have a negative view of the former Massachusetts governor.

That is up from 45% in May.

Romney is under fresh pressure over his unwillingness to release his tax returns from before 2010.

The Washington Post-ABC News poll  also said however that 40% of voters see him in a positive light which is practically the same proportion from the spring.

In contrast 53% said they had a favorable opinion of the Obama challenger while 43% said they felt negatively about him.

By a margin of 53% to 40% more key independent voters see Obama favourably rather than unfavourably.

Romney has had a rough summer marred in part by a series of slip-ups during a trip abroad to Britain, Israel and Poland.