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UK railway infrastructure firm Network Rail's net debt, which is guaranteed by the government, has risen above £30bn.

It rose from £27.28bn in 2011/12 to £30.35bn in 2012/13.

Profit after tax was £699m compared with £761m in 2011/12, and operating profit fell from just under £2.35bn to £2.22bn in 2012/13.

The firm said it faced "challenging" punctuality targets, and is facing record fines from the Office of Rail Regulation if it does not comply.

"Performance is at historically high levels and despite our best endeavours the weight of traffic and extreme weather means that regulatory performance targets for the end of the control period will be very challenging to meet," Network Rail said.

The company, which is responsible for track and signalling, said that 90.9% of passenger trains ran on time in 2012/13, a dip on the 91.6% figure for the previous 12 months.

"We continue to make improvements and aim to raise punctuality to as high levels as possible," it said.

Meanwhile, revenue rose from just over £6bn in 2011/12 to just under £6.2bn in the last financial year.

Network Rail, which is responsible for track and signalling, said that 90.9% of passenger trains ran on time in 2012/13, down from the 91.6% figure for the previous 12 months.

Failure to meet punctuality targets has meant a reduction in the annual bonus of NR directors.

But in figures last week it was revealed that the top five Network Rail executives have been awarded bonuses equivalent to 17% of their annual salaries.

Chief executive David Higgins is to get an extra £99,082 on top of his salary of £577,000.

Releasing its financial results Network Rail said: "Over the last 12 months we have invested an unprecedented amount in growing and expanding the rail network through over 2,000 projects nationwide.

"However, the economic times in which we live mean that alongside delivering new capacity, we need to keep a constant drive for improved efficiency.

"Our overall financial performance remains strong and we are on track to deliver over £5bn of cost savings for the five years to 2014."