LONDON (Reuters) - Newly-privatised Royal Mail Group (>> Royal Mail PLC) will join Britain's top share index following a strong share price performance since its stock market debut, FTSE confirmed on Wednesday, two months to the day after its listing.

The postal firm has nearly doubled its share price since its high profile October 11 listing, with the government forced to defend itself against criticism that it sold the company too cheaply.

Around 40 million Royal Mail shares should change hands in trade worth nearly 250 million pounds ($410 million) in the fortnight around the quarterly reshuffle, analysts said.

It is to be joined in the FTSE 100 <.FTSE> by Ashtead (>> Ashtead Group plc), which will also make its debut in the top share index. The industrial company confirmed its promotion after a 3.7 percent rally on Tuesday when it guided its profit expectations higher for the year.

The companies will replace Indian miner Vedanta (>> Vedanta Resources plc), and specialty chemicals maker Croda (>> Croda International Plc) in the index.

Vedanta will become the fifth stock from the basic resources sector to leave the index so far this year, following Kazakh miners Kazakhmys (>> Kazakhmys plc) and ENRC (>> Eurasian Natural Resources Corporation), steelmaker Evraz (>> EVRAZ plc) and Polymetal (>> Polymetal International PLC) in a year that has seen miners in particular suffer due to weak commodity prices. Vedanta has dropped out after a seven-year stint as a blue chip.

Vedanta and Croda will join the FTSE 250 <.FTMC> index for mid-caps, along with freshly listed Merlin Entertainments (>> Merlin Entertainments PLC) and estate agents Foxtons (>> Foxtons Group PLC). JD Sports Fashion (>> JD Sports Fashion PLC), mobile phone retailer Carphone Warehouse (>> Carphone Warehouse Group PLC), building supplies group Grafton and Riverstone Energy (>> Riverstone Energy Ltd) will also join the index.

Leaving the FTSE 250 are bakery firm Greggs (>> Greggs plc), Hochschild Mining (>> Hochschild Mining Plc), carpet retailers Carpetright (>> Carpetright plc), LED makers Dialight (>> Dialight Plc), newspaper distributor John Menzies (>> John Menzies plc), defence equipment maker Chemring (>> Chemring Group plc) and Schroder Asia Pacific Fund (>> Schroder AsiaPacific Fund plc).

The changes take place after market close on December 20 and take effect from the start of trading on December 23.

(Editing by Andrew Roche)

By Alistair Smout