First Capital Connect cancelled about 50 services on Wednesday
Thousands of rail commuters in south-east England have faced the worst disruption in a week due to a pay row.
Nearly half of First Capital Connect's 400 Thameslink services were cancelled on Thursday and again on Friday morning as drivers refused to work overtime.
The company's plan to freeze pay this year and offer a minimum 3% rise next year has been described as derisory.
Junior Transport Minister Chris Mole called the action irresponsible and asked for it to halt immediately.
The action of the drivers is unofficial, although unions are to ballot for strike action which could affect Christmas travel.
First Capital Connect confirmed that about half its Thameslink services were running on Friday morning.
All services between London and King's Lynn and Peterborough were cancelled on Sunday.
Fifty services in Bedfordshire, Norfolk, Surrey, Sussex, Cambridgeshire and London failed to run on Wednesday.
And the disruption was at its worst on Thursday, with 200 services cancelled, First Capital Connect said.
'Held to ransom'
A spokesman said there had been an "escalation in cancellation in Thameslink services since Monday", forcing the company to introduce a new timetable reflecting the cancellations.
Mr Mole was called to the Commons after an urgent question was put down by the Conservative MP for St Albans, Anne Main, who said commuters were facing "absolute chaos".
Mr Mole said: "Concerted action to stop trains running is irresponsible.
"Train companies need to ensure their staffing arrangements are robust, so they cannot be held to ransom in this way."
"The action by drivers on FCC appears to be coordinated and it is highly regrettable given that talks are continuing."
A statement on First Capital Connect's website read: "We are extremely sorry that a shortage of train drivers has forced us to cancel a large number of services on the Thameslink route.
One commuter decribed the situtation as a joke
"To allow us to operate as consistent a timetable as possible over the coming days, we have introduced an amended timetable."
It added it was in ongoing pay talks with the drivers' union Aslef in a bid to ease the situation.
Aslef's Mick Whelan said the drivers were contracted to work four days a week, and the company had been covering the other three days on a goodwill basis.
He told BBC Radio 4's Today programme: "People reacted and withdrew their goodwill on overtime on an individual basis because of what is seen as a rather derisory pay offer.
"I'd imagine there was a degree of peer pressure but the reality is, it only takes a small amount of drivers based upon the shift system to disrupt the service unfortunately."
He added: "We have been arguing for two years for full employment, we have been arguing for having lots of drivers in place.
"We have been fighting for the right number of drivers so this can't happen."
Liberal Democrat transport spokesman Norman Baker said: "A 1970s-style industrial dispute is the last thing the railways need.
"Whatever differences there are between management and the unions, there is no reason for the passenger to suffer."
Shadow Transport Secretary Theresa Villiers accused unions of "irresponsible game-playing".
"A strike by drivers in the run-up to Christmas would be deeply disruptive and those involved must do all they can to avoid it," she said.