He also worked together with Peter Mandelson on books outlining the political philosophy of the Labour Party under Blair's leadership.

After completing his degrees Liddle worked in research for the Oxford School of Social and Administrative Studies and as an industrial relations Officer for The Electricity Council.

Liddle then moved into the private sector for 10 years, taking the position of managing director of Prima Europe Ltd – a consultancy company advising on the impact of politics and regulation at European and national level.

[2] During a seven-year spell in this role he developed a new UK policy of positive engagement in the European Union, focusing on economic reform, innovation and enterprise promotion; modernisation of Welfare States and labour market reform; as well as institutional issues, the Constitutional Treaty, and European Defence.

Advancing from Number 10, he became a Member of the Cabinet of the European Union Trade Commissioner, where he advised on EU policy and their impact on the UK.

Liddle is currently chairman of Policy Network – an international think tank bringing together academics, policymakers and politicians across the progressive centre left; responsible for major projects on the future of the European Social Model, public service reform, immigration and integration, flexicurity, and globalisation and social justice.