The head of the Royal College of Nursing has apologised after nurses in England complained of being "misled" over a new pay deal.
The union had said they would receive a 3% pay rise this month as part of a new three-year pay deal.
But a number of nurses took to social media to say their pay had gone up by much less - in some cases by pennies.
RCN general secretary Janet Davies apologised, saying the deal had been presented "in good faith".
In June, 13 unions agreed to a three-year pay deal for more than one million NHS workers in England.
Only one union, the GMB, rejected the deal.
At the time, the then Health Secretary Jeremy Hunt said the pay rise was "incredibly well-deserved for staff who have never worked harder".
Nurses had been told by the RCN they would get a 3% pay rise in their July pay packets, backdated from April.
But this week, some took to Twitter to complain that the increase they had expected had not materialised.
Many expressed anger at the RCN and the government about the deal, with some nurses saying they felt they had been misled by their union.
Ms Davies responded by offering a "sincere personal apology".
"I'm as dismayed and angry as you are and I will fight the corner of members at every turn," she said.
"In good faith, we told all members that they would receive a 3% uplift this summer.
"I now find that this is not the case for everyone. I can assure you that I am demanding answers for you."
The RCN added in a statement that it would not be reopening the deal.
"Despite some delays to payments, over the three years the deal has to run, members will receive the full amount promised," a spokesman said.
"We are sorry for any confusion caused about what members were due to receive this month."
However, NHS Employers, which agreed the pay deal with the unions, said the agreement had never promised a 3% rise in the first year.
"This miscommunication is very unfortunate and clearly the RCN will need to review all of its communications to understand the extent of its error.
"We hope that this issue between the RCN and its membership can be resolved quickly, and we would direct colleagues towards the information on our website, which makes clear the pay journeys for different staff over the next three years."