A three-day strike by more than 2,000 Tube maintenance staff is set to start on Sunday evening, it has been announced.

The action, by engineers who maintain two-thirds of the Underground, would have an "immediate and massive effect", The Rail, Maritime and Transport (RMT) union said.

Last-ditch talks to resolve the dispute over moving 250 jobs from the Tube maintenance firm Metronet to the train maker Bombardier resumed this morning.

The RMT accuses Metronet of breaking a promise not to transfer staff to other companies.

But Metronet says it has "bent over backwards" to reduce the transfer from 700 staff to 250 posts - mainly managers and technical specialists.

They are due to work on 237 new Tube trains that will be supplied and maintained by Bombardier from 2009.

Earlier this week the RMT's ground-level staff representatives accepted the deal, along with Amicus, the other union involved. But RMT bosses then decided to override the decision. "We don't understand why," said Metronet senior vice president Mark Cooper.

The staff transfer would improve services, he added. Work done by Bombardier and Metronet on the Central line fleet have made those trains three times more reliable.

Mr Cooper urged the RMT to resume talks.

"The planned strike action will lead to disruption for the three million people who use the Tube every day. It is unnecessary and unwarranted."

About a third of the RMT members entitled to vote supported the strike.

The union also announced that an indefinite overtime ban would commence at 6pm next Sunday, April 22.

RMT general secretary Bob Crow said: "We have had a series of meetings with Metronet this week but we have been unable to reach agreement."

London Underground said the strike would involve only maintenance staff - no drivers or station workers.

"The Tube will not come to a standstill in the event of industrial action of this type," an LU spokesman said. "But the cumulative effect of any action would be negative depending on the duration of any dispute."

Strike condemned Meanwhile the passenger group London TravelWatch condemned the proposed strike.

The watchdog's chair Brian Cooke said: "This strike is pointless and will achieve nothing for the staff involved; it is just the long suffering tube passenger that will be greatly inconvenienced."

If the RMT wants to oppose the Public Private Partnership (PPP) contracts on the Tube, they "should attack the Government that set the deal up, not the innocent commuter trying to get to work to earn a living", Mr Cooke added.

"Striking over the transfer of a comparatively small number of employees from one privatised company to another will not change anything."

Roger Evans, the Conservative deputy chair of the London Assembly transport committee, also criticised the strike: "Once again Londoners are facing strike action and transport chaos.

"Instead of staying at the negotiating table with Metronet, Bob Crow and his cronies have shown their lack of maturity and short-sighted aims by punishing the very people who ultimately pay their wages."