A pledge to double the length of trains between King's Lynn and London hang in balance despite promises the new-look service would be in place by the end of the year.

There are currently eight-carriage Great Northern trains serving the Ely and Cambridge stations but officials at a rail summit in January last year hoped to extend the service to Watlington, Littleport and Waterbeach by December 2018.

But in Network Rail's strategic business plan for Control Period 6 - the next round of funding for projects in the next five years - states that the final investment decision for longer trains has not yet been decided.

A spokesman from Network Rail said this is an internal governance process which every project has to go through, adding: 'The King's Lynn eight car scheme is amongst the CP5 projects that have funding.

'We are currently working up plans with operators to confirm when the infrastructure works to enable these services can take place.

'Once we have agreed access arrangements, a detailed construction programme will be developed which will allow us to notify our stakeholders and the wider community about the expected commencement of the works.

'We should be in a position to set out expected delivery timescales for these infrastructure works once we have completed our usual internal planning and governance processes.

Published earlier this month, the strategic business plan outlines how it will spend more than £2b to improve train services in East Anglia between 2019 and 2024.

It states that the King's Lynn eight car project has been included under the Ely Area Capacity Enhancements scheme, which also incorporates improvement works for the bottleneck Ely North junction.

The spokesman added: 'The developing work for the Ely area is an enhancement scheme and would therefore require an additional funding stream to be identified.

'The strategic business plan, in summary, is our proposal for maintaining and operating the railway from 2019 to 2024 and does not include enhancement schemes.'

Colin Sampson, chairman of Fen Line Users Association (FLUA) said plans for longer trains in Lynn was originally a stand alone project which does not need Ely North improvements in order to go ahead.

He said work on the three stations have been costed and agreed by Network Rail engineers, with Watlington and Littleport to use selective door opening and the Waterbeach platforms to be extended.

But he added: 'Being agreed is one step short to being spent. I do not see the work beginning this year, unless we see a shovel in the ground or a train running there is always a chance it is not going to work.'