Political parties north of the border have promised to crack down on cheap supermarket alcohol and push local sourcing if they are voted in at next month's Scottish parliamentary elections.

The SNP, which opinion polls indicate will be the largest party after the elections, is pledging a "tough clampdown on irresponsible drinks promotions, including action to stop the deep discounting".

It said it would extend legislation curbing on-trade promotions to the off-trade. "It is not acceptable that water should be more expensive than alcohol," it said.

The Lib Dems pledged to introduce curbs on in-store alcohol deals and to consider a mandatory 'no proof, no sale' scheme to stop sales to minors if test purchasing proves ineffective.

All the major parties have proposed plans to boost local sourcing, particularly through public sector procurement contracts. But the SNP has gone a step further by promising to consult on changing planning regulations to include a requirement that supermarkets stock a certain proportion of locally sourced goods.

The Conservatives, meanwhile, said they would "work with Westminster to attempt to have the supermarkets code of practice strengthened so that local producers receive a fairer price". They would also offer incentives to producers to form co-ops to enhance bargaining power.

The Lib Dems promised public sector support for a private sector-led Scottish food and drink marketing body modelled on Ireland's Bord Bia. Labour proposed an initiative to get empty high street shops back in use. It said it would establish Town Centre Trusts, backed by a £50m Turnaround Fund, to enable people to set up community stores in vacant outlets.

The trusts would be given powers to initiate compulsory purchase orders.

Polling takes place on 3 May and with no party likely to win more than 50 of the 129 seats, a period of negotiation on policy is expected to follow.