Rail firms came under fire this weekend for planning a series of fare hikes of up to 87 per cent just ten days after the general election - despite the main party leaders pledging to freeze prices. The Prime Minister has requested First Great Western (FGW) 'urgently review' its decision to almost double some fares on the route between London and the Cotswolds - which passes through his Witney constituency. Mr Cameron has promised to freeze fares in real terms for the whole of the next parliament, while Labour has also pledged to bring in a 'cap' on price hikes. Scroll down for video . Sent letter: Prime Minister David Cameron has written to First Great Western to request it 'urgently reviews' plans to increase fares on trains running through his Witney constituency . Passengers travelling from Charlbury, the Oxfordshire station nearest to his constituency home, can currently purchase an off-peak return to London costing £33.30. This allows them to leave at 9.42am, outside the morning peak, but return on one of the four services that go from London during the evening peak period. But from May 17, these customers will be forced to pay £62.40 if they want to come home on one of these trains - an 87 per cent increase. The planned changes have angered Mr Cameron's constituents - prompting him to personally intervene. 1. First Great Western . Line: London to Hereford . Potential increase: 87% . 2. Virgin . Line: Stafford to Liverpool . Potential increase: 24.5% . 3. London Midlands . Line: Crewe to Northampton . Potential increase: 23% . The Prime Minister's spokesman told The Sunday Times: 'Mr Cameron has written to First Great Western and asked them to urgently review their decision.' Train companies have three windows every year in which they can increase 'unregulated' fares, which make up around half of their total revenue. Unregulated fares can include journeys such as off-peak leisure and advance-purchase tickets. First Great Western has insisted the reason for the change is to bring the north Cotswolds route in line with its other services. The firm is also offering some customers a cheaper option in advanced single tickets, potentially allowing them to buy a return for £20 if they book ahead. Other sharp price hikes to be introduced include an off-peak first-class single from Crewe to Northampton on London Midlands, which will rise from £30 to £37, according to trainsplit.com - a website that provides passengers with cheaper ticket options. The cost of an anytime return between Stafford and Liverpool, travelling with Virgin, will leap from £26.50 to £33 - a 24.5 per cent hike. Biggest hike: Passengers travelling with the rail firm between London and the Cotswolds could face increases of up to 87 per cent come May 17 . And an anytime first-class day return on Virgin from Manchester to Stoke will go up 22 per cent - rising from £29 to £35.30. Virgin has said the 'vast majority' of its price hikes affected 'Virgin Trains-only fares' - which are at least 20 per cent cheaper than tickets offered by rival firms. It added: 'Our overall fare increase this year on the west coast is just 1.65 per cent, well below the industry average.' Speaking to Mark Hookham of The Sunday Times, Mike Richardson, of trainsplit.com, said: 'Passengers might very well be surprised at these fare increases when both the main parties have talked about price freeezes. According to Which? Hundreds of 'anytime' and 'off-peak' returns rose by 40 per cent in January. Its research found rail fare increases on tickets that passengers buy on the day and on rural routes. Defended rise: Virgin has said the 'vast majority' of its price hikes affected 'Virgin Trains-only fares' - which are at least 20 per cent cheaper than tickets offered by rival firms .
PM has written to First Great Western asking for 'urgent review' of fare hike .
Passengers in his Witney constituency could face increases of up to 87% .
Comes after Mr Cameron pledged to freeze fares for whole of next term .
Other sharp hikes include Virgin's service between Stafford and Liverpool .