The Conservatives and Liberal Democrats have called for the state second pension to be scrapped after insurers set out plans to write to six million people advising them to reconsider their decision to opt out of the government-run system.

They said the letters were further evidence that the state pension system was too complex for ordinary people.

David Willetts, the Conservative pensions spokesman, said: "We've long argued that the S2P rebates aren't set at a fair level. Now it seems the insurance industry can no longer justify collecting rebates for people who simply won't benefit from them."

Steve Webb, the Liberal Democrats' pension spokesman, added: "This highlights how confusing the business of contracting out has become."