He resigned last year after it was revealed that he had received a £373,000 loan from fellow minister Geoffrey Robinson, who also left the government.

Speaking at the Amalgamated Engineering and Electrical Union conference in Jersey, Mr Mandelson said: "You can imagine how I felt these last six months - a bit disorientated, a little awkward, not knowing quite what to do with myself."

But he said he did not miss the trappings of office, although he confessed to having become used to some of its perks.

"I have only recently got out of the habit of jumping in the back of a car and wondering why they don't move off," he said.

Making a difference

Mr Mandelson said his greatest regrets centred on a sense of missed opportunities.

"What I really miss is the chance to make a difference," he said.

But the main focus of his speech was his strong backing for joining the euro - once the chancellor's list of five economic conditions have been met.

"Of course there is a loss of monetary sovereignty were we to sign up for the euro and I understand that for some people the currency is a potent symbol of national identity.

"But the issue is whether the gain from joining the euro is greater than the loss."

Jobs and investment

Laying out what he said are the benefits of joining up to the single currency, Mr Mandelson said it would preserve British jobs and boost investment.

"We must get across this simple message - British jobs and prosperity increasingly depend on Europe.

"It makes sense to share the same currency as our largest market, especially when the rest of Europe has already taken this step because it will enable us to trade more.

"If the euro is the success that many predict, life outside it could be pretty uncomfortable for Britain," he added.

Turning again to his time in the Cabinet, Mr Mandelson said he was proud of the time he spent at the Department of Trade and Industry which had seen him implementing the national minimum wage and putting new rights for workers into legislation.

Mr Mandelson told delegates that his true friends had always been in the Labour Party and in the trade union movement.

"If I did not realise it once, I certainly know it now," he said.

'Class act'

Speaking after Mr Mandelson's speech the AEEU's General Secretary Sir Ken Jackson said: "Peter is a class act and should return to the front line of politics.

"He confirmed his support for the single currency but also confirmed that he is one of Labour's greatest assets.