The BBC's coverage of chancellor George Osborne's spending review on 20 October faces disruption after broadcasting unions today announced strike dates in their dispute over pension scheme changes.

The National Union of Journalists, Bectu and Unite today rejected BBC director general Mark Thompson's latest revised pension reform offer, communicated to staff by email earlier in the day, and confirmed two 48-hour strikes designed to cause maximum disruption to the corporation's coverage of key political events this autumn.

They intend to strike on 5 and 6 October, which would disrupt coverage of the final two days of the Conservative party conference, and 19 and 20 October, to coincide with Osborne's announcement of the scale of government cuts in the Treasury's public spending review.

A planned strike for 20 and 21 September, which would have impacted coverage of the Liberal Democrat party conference, has been called off.

The unions now intend to consult with their members, holding a series of meetings with BBC staff across the UK on the pension proposals until 30 September. There will then be a joint meeting of the three unions' officials on 1 October.

Calling the strikes is a hard-edged negotiating tactic to put pressure on the BBC to offer further concessions to its pension proposals.

"The BBC's continued insistence that staff pay much more for significantly worse benefits is unacceptable," said Jeremy Dear, general secretary of the NUJ. "We will be taking the union's case for fair pensions to a series of members' meetings across the UK. If the BBC fails to listen to the continued anger of staff at these unacceptable pensions changes we will be left with no choice but to strike to stop the pensions robbery."

Earlier today Thompson warned staff that BBC management did not have much scope to negotiate in attempting to overhaul the corporation's pension scheme to try to plug a deficit that could be as much as £2bn.

"Pension reform at the BBC is inevitable," he said. "We have a large current deficit in our scheme and face long-term pressures which, without action now, would undermine the scheme and compromise the broader finances of the BBC."

The main concession offered in Thompson's email today is a new career-averaged benefits option – "CAB 2011" – for existing BBC pension scheme members. Pension benefits would be based on an employee's average pay from the time they took this option to when they left the BBC.

Previously management had been offering staff two options – stay in the final salary scheme but have future salary increases for calculating pension benefits capped at 1% annually, or move to a new defined contribution scheme.

This 1% cap provoked outrage from long-serving and in some cases senior BBC employees, who risked seeing their final salary pension benefits severely curtailed.

Another significant concession is protecting the value of pension benefits that employees have accrued thus far in their career.

However, the new CAB 2011 scheme will require staff to contribute more, 7% of their salary, up from 4% under the current career average scheme.

The current pension scheme costs the BBC about £140m per year. However, the corporation has estimated that this figure will balloon to about £350m per year when it takes into account factors such as dealing with deficit repayments, the scale of which will be properly known in April next year when the pension regulator determines the exact size of the deficit.

"Whilst the BBC has heard and responded to some of our concerns, we are disappointed that the BBC's response fails to take account of the long-term implications of the worsening of pension terms for staff and their families," said Gerry Morrissey, general secretary of Bectu. "We believe that the BBC should be prepared to justify its position fully by agreeing to revisit pension benefits in April 2011 should official figures confirm that the total scheme deficit is less than £1.5bn. That said, the latest proposals from the BBC warrant further detailed examination with members."

The BBC reckons that the current deal on the table, including the CAB 2011 option, will cost it about £250m a year – an overall saving of £100m on the projected £350m a year future cost of the current scheme. The BBC has to pay off the deficit with 10 years.

The BBC said : "We are disappointed that the unions have announced dates for industrial action. We are surprised that they have done so when we have just announced today a new 60 day consultation period on a significant addition to our pension proposals.

"We believe our pension proposals address concerns raised by staff and that they should now have the opportunity to fully consider and explore them within the context of an ongoing consultation rather than the threat of a strike."

Any shortfall in repayments not covered by savings made from overhauling the current scheme will have to come from other areas such as cost-cutting or even asset sales.

Peter Skyte, national official for Unite, said that while the BBC's latest proposals went "some considerable way to continuing the link between basic pay and pension increases, they fall short in some areas and require clarification in others".

"The BBC has a world class workforce which deserves world class pensions to provide for surety and security in retirement," he added. "We feel it necessary to consult further with our members on the BBC's latest proposals before determining a response and further action."

• To contact the MediaGuardian news desk email editor@mediatheguardian.com or phone 020 3353 3857. For all other inquiries please call the main Guardian switchboard on 020 3353 2000.

• If you are writing a comment for publication, please mark clearly "for publication".