NHS Direct Chief Executive, Nick Chapman, denied the service was at breaking point and said there had been no increase in calls related to flu or colds.
He told BBC Radio 4's Today programme: "We are not at breaking point. We continue to deal with large numbers of patients to give them advice. We are providing a good service, not as good as we would like, despite the efforts of our staff."
Mr Chapman went on: "I think it is right that because we are not able to provide the normal service, I think it is right I should apologise."
But he said only 4% of enquiries related to colds or flu: "We don't detect any particular upsurge in colds and flu."
He blamed the increase in enquiries on the cold snap meaning people were more reluctant to leave home to consult their doctor.
And he conceded that with the 50% increase in enquiries: "We find that is very difficult to cope with."
A spokeswoman insisted the health service was coping "very well" with the outbreak of flu.
She said: "We are currently seeing an unusually high number of people in critical care with flu. But the NHS is well prepared.
"There is always more pressure on the NHS at this time of year and this year is no different.
"But the NHS is coping very well with only a small percentage of the intensive care capacity being taken up by patients with flu.
"However, as a cautionary approach, local health trusts are looking at how they can increase capacity if necessary.
"Prevention is better than cure - it's very important that people who are at risk have their free seasonal flu vaccination. All pregnant women should have the flu jab - at any stage of pregnancy. If you are eligible contact your GP or practice nurse now to get your seasonal flu jab.
"If people are at all concerned they can call NHS direct or speak to their GP."