Given that the Lib Dems went into the election promising an amnesty for illegal immigrants and ended up supporting the Tories' unworkable cap, it's hardly surprising that Vince Cable feels the need to reassert his liberal credentials.

Cable's declaration that he wants to see as "liberal an immigration policy as it's possible to have" has succeeded in bringing the cabinet's internal divisions out into the open. Pushing the principle of collective ministerial responsiblity to the limit, he revealed that he was "arguing within government" for "the most flexible regime possible".

But as the Spectator's Fraser Nelson points out, the fact that David Cameron's pledge to reduce net migration to "tens of thousands" a year was not included in the coalition agreement means that the policy is up for negotiation -- and rightly so.

Immigration fell significantly during the recession, but net migration of 142,000 in 2009 indicates that Cameron would need to cut immigration by at least 30 per cent to bring the total to less than 100,000. Privately, Tories speak of an even more unrealistic target of 50,000.

Cameron's promise remains unfeasible for several reasons. For a start, the government cannot limit immigration from within the EU without restricting the free movement of labour and throwing the UK's continued membership into doubt. The policy also ignores the 39,000 people who come to the UK on spousal visas after marrying British citizens abroad.

In the case of his India trip, Cameron's declaration that "Britain is open for business" sits uneasily with his belief that the door must be closed to some. Cable may be aware of this, but his call for a "flexible cap" -- a contradiction in terms -- reveals the tangle the government has got itself into.

The most practical and liberal policy would be to abandon the cap altogether but, for now, it looks like the coalition will try to muddle through.