A unofficial "wildcat" strike by postal workers yesterday has left an estimated 10 million letters undelivered.

The strike was called by union members in 15 postal centres acroos England in sympathy with colleagues in Watford. The 800 staff at the Watford sorting office walked out last week in protest at changes in shift patterns.

The centres affected include Liverpool, Stockport, Chester, Manchester, Preston, Maidstone, Cardiff and all five main sites in London.

Yesterday, talks to prevent the strikes from spreading continued between executives from Consignia, the new name for the Post Office, and representatives from the Communication Workers Union.

A Royal Mail spokesman said yesterday that "It is very regrettable the effect this is having on our customers".

One processing worker on strike in London echoed the regret but said: "We have to make a stand."