THE UK government has "neglected" south Wales' rail system, a group of Welsh MPs have claimed.

A joint letter signed by 22 Labour MPs has been sent to transport secretary Chris Grayling and Welsh Secretary Alun Cairns claims Wales is being "left behind" as a result of a lack of investment in rail.

Among the signatories are Newport's Paul Flynn and Jessica Morden, Torfaen's Nick Thomas-Symonds, Islwyn's Chris Evans, Blaenau Gwent's Nick Smith and Caerphilly's Wayne David.

This follows last month's announcement that long-promised electrification of the line between Cardiff and Swansea had been cancelled.

In the letter the MPs claim the region's rail network has suffered "continued neglect" by the government.

"We believe your unwillingness to recognise funding gaps in the network is causing Wales to be left behind and we call upon you to address the situation urgently with additional investment," it read.

"The recent announcement that you will be reneging on your promise to electrify the Great Western Mainline from Cardiff to Swansea forms part of a series of failures to provide adequate funding for transport in our region."

The letter also claimed only 1.5 per cent of the UK government's total investment in rail had gone to Wales since 2011, despite 11 per cent of the country's rail system being in Wales.

It said: "We are calling on you to stop letting Wales down and call on the government to reverse the decision to renege on electrification to Swansea, address the historical underfunding of the Welsh railway network and honour the funding agreement you made with the Welsh Government in 2014 to secure a sustainable new Wales and Borders franchise agreement."