* 42,000 mail centre staff and drivers to strike Thursday
* 78,000 delivery and collection staff to strike Friday
* Further strike dates to be announced this week (Adds Royal Mail comment, detail)
LONDON, Oct 21 (Reuters) - Negotiations failed on Wednesday to prevent a strike by Britain's postal workers this week and their union leaders said there will be further walkouts.
Thousands of staff working for the state-owned Royal Mail are due to walk out on Thursday and Friday in a row over pay, jobs and modernisation.
The action is expected to severely disrupt mail deliveries.
Negotiations between the company and the Communication Workers Union made progress but were undermined by senior Royal Mail bosses and Business Secretary Peter Mandelson, the union said.
"Unfortunately the situation is the strikes will go ahead," said Dave Ward, the CWU's deputy general secretary, saying further strike dates would be announced later this week.
"What we've seen in the last few days is, in our view, deliberately being choreographed in a way that tells us that the government and Royal Mail are working hand in hand to avoid any chance of us reaching a solution."
Some 42,000 mail centre staff and drivers are due to strike on Thursday followed on Friday by 78,000 delivery and collection staff.
Royal Mail's business has suffered in recent years as consumers and industry switch to the Internet or more specialised postal services.
Royal Mail and the government have warned the strikes will damage the company, put small firms that use its services in jeopardy, and push clients to competitors.
The company's Managing Director Mark Higson condemned the strikes as wholly unjustified.
"The union has yet again failed to honour its commitment to call off strikes in return for a period of no change and has shown yet again that its intention is to inflict as much damage as it can on the postal service," he said.
Mandelson called for more talks, saying Royal Mail had to modernise to survive and its finances would be pushed into the red by the strikes.
"National strike action is not in the best interests of the company, the workforce or the hard-pressed consumers and businesses that depend on Royal Mail," he said.
The government wants to sell up to 30 percent of the company to make it more competitive, but set aside those plans earlier this year due to adverse market conditions and opposition from workers and Labour politicians.
Royal Mail has said it would hire 30,000 temporary staff to help cope with the backlog expected to be created by the strikes and a higher Christmas workload -- double the normal seasonal intake. (Reporting by Michael Holden; Editing by David Cowell/Will Waterman)