Liverpool legend Jamie Carragher says John Terry's form has been key to Chelsea's title challenge.

The Blues captain has threatened to retire at the end of the current campaign if he is not handed a new deal by the west London club.

"Gary Cahill and Kurt Zouma are good players but without Terry, I would not be confident about Chelsea winning the Premier League next season," the Liverpool legend wrote in the Daily Mail.

"If he was to leave, there would not be a team in the top flight who Terry would not improve. Mourinho knows this better than anyone.

"Terry was as good against Tottenham (in Sunday's Capital One Cup final) as he was against Liverpool at the Millennium Stadium in 2005 and you have to ask when the time comes to fill his shoes, what will the price be?

"When you think Manchester City paid £32million for Eliaquim Mangala last summer and David Luiz cost Paris Saint-Germain £50m, you get an idea of how difficult it will be to find someone with Terry's many qualities."