The decision, which was made by a committee of market experts yesterday, will become effective after market close on December 19.

The companies’ shares have gained 30 per cent and 20 per cent respectively this year on the back of Britain’s rising property market.

They will replace Petrofac and IMI on the London Stock Exchange.

The LSE said on Wednesday that the six companies on the FTSE 100 reserve list, which is used in the event of a merger, delisting or suspension, would be Amec Foster Wheeler, Hikma Pharmaceuticals, Inmarsat, Investec, Merlin Entertainments and Pennon Group.

Companies entering the FTSE 250 - often regarded as a better barometer of the state of British business than the multinationals on the blue-chip index - will be bakery chain Greggs, newly floated shoe firm Jimmy Choo, and retailer Game Digital, along with investment firm Allied Minds, property group CLS Holdings and Spirit Pub Company.

Foxtons is one of the companies falling out of the FTSE 250. Shares in the estate agent have fallen 44 per cent since floating in September 2013.

Others losing their place include EnQuest, Fenner, Ferrexpo, Hochschild Mining and Spirent Communications.