More than 4,000 designs were received from 526 designers.
The new coins are 0.2mm wider than the existing 1.7mm version and are being made from cheaper materials in a bid to save £10m a year.
Industry bodies warned of a "disaster", saying it was impossible to alter millions of vending machines and 400,000 parking meters to accept the new coins in time for their slated introduction from 1 January.
Now the Treasury and the Royal Mint say the coins will not be introduced until at least April.
Jonathan Hilder, chief executive of the Automatic Vending Association, said: "We're relieved the coins have been put back to April and we are hopeful there could be a further delay until next January."
"This has already saved the industry £16.3m in costs and will prevent millions of machine users having coins rejected."
He added: "The Treasury has been much more responsive to the needs of the industry than under the previous government."
The current coins – made from nickel and copper alloy, cupro-nickel – will be replaced by those made from nickel-plated stainless steel in a bid to save money as global copper prices are driven up by demand from China and India.
A spokeswoman for the British Retail Consortium said: "Low-cost and convenient parking is an essential part of having a successful high street.
"If changes have to be made to vending or parking machines it is very important enough time is allowed so that everything will be working effectively. Retailers don't want customers to face any inconvenience that could have been avoided."
A spokesman for HM Treasury said: "It is intended that the new 5p and 10p coins will be issued with natural demand from 1 April 2011, saving £10m a year in reduced metal costs."
A Royal Mint spokesman said: "There are many benefits of introducing nickel-plated coins. Nickel-plated coins last longer, with a typical lifetimes of 25-30 years and are cheaper to manufacture."