A rail ticketing app set up by a pair of entrepreneurs frustrated by the lack of legroom in standard-class carriages will this announce a multimillion pound fundraising backed by a syndicate of blue-chip investors.

Sky News understands that Seatfrog, a privately owned rival to the London-listed company Trainline, has secured £6m in new backing from shareholders including Pembroke, a venture capital firm which has backed successful UK companies including the fashion brand Me + Em and Five Guys, the burger chain.

Octopus Ventures and Praetura are also participating in the funding round, which is expected to be announced publicly on Tuesday.

It was unclear at what valuation the new capital had been raised.

Seatfrog, which says it achieved revenue growth of over 800% last year, was launched in 2018 by chief executive Iain Griffin and Dirk Stewart.

Its initial target was to address the 570m first-class seats remain empty on British rail journeys every year.

Since launching, it claims to have saved passengers more than £43m by helping them to upgrade or switch trains.

There has been persistent unhappiness for many years among consumers about the cost and complexity of British rail fares, with operators in the privatised industry routinely accused of gouging passengers and offering poor reliability and service.

Seatfrog's co-founders Dirk Stewart (l) and Iain Griffin launched the company

Seatfrog says its technology benefits consumers, but also train operators by helping them to grow revenues from higher-value seats.

It has struck partnerships with 16 train operators covering more than 3bn journeys across the UK and Europe annually.

The company's most recent alliance has been with the Italian operator Trenitalia.

"Our mission is to be the platform of choice for rail, helping people fall in love with taking the train again, and enabling operators to run more profitable networks full of happier, less stressed passengers - it's long overdue," Mr Griffin said.

"With the launch of our Train Swap feature across the UK and our expansion into new European markets, we are unleashing a new platform that helps passengers get an unrivalled experience and operators run more efficient networks well beyond the capabilities of legacy technology."

Fred Ursell, investment director at Pembroke Investment Managers, said the lack of innovation in the rail sector was among the key factors in British customer dissatisfaction and hailed Seatfrog's "disruptive business model, product market fit and large market combined with a world-class team".

Rival app Trainline listed in London in 2019, having been owned by a number of private equity firms.

It now has a market value of close to £1.2bn.

finnCap advised Seatfrog on the funding round.