Pressures on funding of Network Rail's £40.5bn Control Period Five have been brought into stark view by a letter leaked to The Independent which says spending cuts will be needed if a £1bn track renewals programme is to continue as planned.

The letter, penned by the director of Network Rail's high output track renewals division, Ben Brooks, says significant overspend in some renewals projects, plus higher costs than expected in route operations, means "there is simply not enough money left in CP5 to continue as we have been".

High output plain-line track renewal projects are carried out by Network Rail's factory trains and represent roughly one quarter of Network Rail's £800m/year track renewals programme across all of Britain. The £4bn total track renewals spend marked out for the 2014-2019 CP5 also includes about £1bn of plain-line renewals that the factory trains cannot get to, plus around £2bn of switch and crossings work carried out by Network Rail's contractors.

According to The Independent,  Brooks' letter says the overspend had already caused a significant number of renewals projects to be postponed, including all work on the London North Western, Wessex and Wales Routes until after 2019. The result is that "the whole of Network Rail has to cut costs", the letter says.

Unions have reacted angrily, pointing to threats to safety due to project cuts, but Network Rail has denied any danger to rail safety: A spokesman said: "Network Rail has no plans to cut any safety-critical jobs or any safety critical work. Any such suggestion is ridiculous scaremongering.

"The rail network has faced a number of cost pressures and changing priorities, and like all businesses we have to live within our means. This sometimes means making hard choices.

‘We are making savings in non-essential areas to ensure we’re delivering value to the taxpayer, while at the same time continuing to deliver a safe, reliable and expanding railway. The safe running of the rail network will never be compromised and the adjustment in the activity of our high output track renewals work represents just 0.3% of our spend in the running of the railway network."