The latest growth initiative from George Osborne is a state-backed small business bank. The Chancellor said over the weekend that the difficulties small businesses face when trying to get the credit they need to keep going or to expand is one of the biggest problems holding back the UK economy. He has already tried to ease this problem with "Project Merlin" (lending targets for commercial banks), a national loan guarantee scheme and most recently the "funding for lending" initiative. Depending on your option, his latest idea can be seen as building on these previous schemes, or an acceptance that they were not up to the task of getting credit flowing in the economy.

But will it work? That will depend very much on how ambitious the Chancellor chooses to be – and the first signs are not encouraging. The bank is being described as a "one-stop shop": bringing together in one place all the various schemes and initiatives designed by government to help small businesses. No doubt this will be helpful for small businesses, making it easier for them to find a way through the Whitehall maze. But what small businesses really want is an increase in the amount of credit available to them and a reduction in the cost of that credit. It is not immediately apparent that the Chancellor’s new bank will deliver on these aims.

Other countries have national investment banks of varying descriptions, including the KfW in Germany and the Small Business Administration in the United States, and the Chancellor’s idea seems most closely modelled on the latter. But importing wholesale the model of any one overseas bank is unlikely to be the best approach.

What we need in the UK is a fully-fledged British Investment Bank designed to suit the particular circumstances of our economy. This Bank should be set up to tackle two longstanding problems: a tendency to invest less in infrastructure (as a share of GDP) than comparable economies and a shortage of financing, particularly long-term financing, for small and medium-sized businesses.

There are a number of important questions to be addressed before such a Bank could set up – and, like the Green Investment Bank, it would need to secure EU state aid approval - but some of the parameters should be clear. The Bank would be 100 per cent state-owned. Its remit would be to increase lending for infrastructure and to SMEs. And its governance structure would have to ensure there was a clear dividing line between where the role of the government ended and the activities of the bankers began.

More controversial would be the capitalisation of the Bank and its ability to raise additional funds in the capital markets. The Green Investment Bank will have an initial capitalisation of £3bn and will not be able to borrow money at least until the government debt ratio is on a downward trajectory (because its activities count as part of the public sector). The same consideration would, no doubt, prevent the current Chancellor from creating a fully-fledged British Investment Bank.

But there is a qualitative difference between the government having to borrow because its current spending commitments are greater than the sums it is prepared to raise in taxes and a BIB raising funds in asset markets to use to finance infrastructure projects that will generate a stream of income in the future, or to lend to small businesses. A British Investment Bank should not be held back by the vagaries of the UK’s accounting practices. Its activities (and those of the Green Investment Bank) should be excluded from the government’s target fiscal measures and it should be free to raise funds up to a pre-determined leverage ratio

The government would, though, have to provide the new Bank with its initial capital. One option would be tell the Bank of England to do another round of quantitative easing specifically for this purpose. Alternatively, the funds would have to be found by cutting other spending, increased taxation, the sale of government assets or extra borrowing. Given the Chancellor’s unwavering adherence to his fiscal plans, this is likely to be a stumbling block to any hopes of a British Investment Bank in the next few years.

And this is now the biggest problem facing the UK economy. Because the Chancellor will not spend more money boosting aggregate demand in the economy, whether directly through infrastructure spending or a temporary tax cut or indirectly by capitalising a British Investment Bank with a directive to lend to small businesses, he is reduced to indirect schemes like funding for lending or the pension infrastructure plan. These require shifts in behaviour by the banks and pension schemes if they are to work. Unsurprisingly, they are not as effective as more direct approaches.

The Chancellor’s state-backed small business bank fits into the same pattern. It is a half measure, bringing together existing initiatives, rather than the creation of the fully-fledged British Investment Bank that the economy really needs.

Tony Dolphin is Chief Economist at IPPR