Labour's second term in power has had its share of disappointments not least because of Iraq, Brendan Barber said.
Mr Barber was speaking at this year's TUC conference in Brighton
In a speech to delegates at the TUC in Brighton the organisation's general secretary said unions still had "unfinished business" with Labour.
Speaking ahead of Tony Blair he said they must make sure pledges won by unions were delivered in a third term.
"Love-hate relationship could have been invented to describe how we get on [with the] government," he said.
Mr Barber stressed the importance to workers of trade union membership and highlighted some of successes of the movement in the past year.
"There is no great secret about why this has been a year of real advance for our movement - it is because we have worked together," he said.
He said during Labour's second term there had been a lack of a comprehensive programme for the workplace "to deliver our vision beyond full employment to quality employment for all.
"We did have a joint programme in the first term and made progress together - the minimum wage, union recognition, the new deal, signing the social chapter - you know the litany.
"But we did not secure an agreement for the second term. We must change that for the third term."
More at ease?
Mr Barber went on to stress the significance of the deal hammered out at the Labour National Policy Forum in Warwick when a 56 point plan was agreed covering a range of issues including workplace rights.
"I think that the programme put together in Warwick has made us more at ease with each other than for some time," he said.
"Best of all it has given us a new sense of common purpose and a confidence that we can make sure of progress towards our goals."