SCOTLAND'S first gay marriages could be held as early as July as part of a collaborative effort with Westminster, SNP ministers believe.

The bill legalising gay marriage in Scotland is expected to be passed overwhelmingly by MSPs in a free vote at Holyrood on Tuesday.

However, putting it into effect will require help from the UK Government, as equality law reserved to Westminster must also be amended to ensure Scottish celebrants cannot be forced to conduct ceremonies against their will.

The Marriage and Civil Partnership (Scotland) Bill will legalise civil and religious same-sex marriages, with an "opt-in" procedure for faiths and celebrants willing to conduct ceremonies and legal protection for those who object.

It also relaxes the rules on where civil marriages can take place, and establishes non-religious or "belief" marriage ceremonies.

Health Secretary Alex Neil, who has piloted the law through Holyrood, will tell Parliament he expects the first same-sex marriages will take place in Scotland by the end of the year.

However, behind the scenes, ­Scottish Government officials hope the process can move at a far quicker pace. It is understood that if a Section 104 Order under the Scotland Act is put through ­Westminster at full speed, it would enable same-sex marriages in ­Scotland by July.

As Westminster legalised gay marriages in England and Wales last year, it should be relatively simple and uncontroversial to amend the law for Scotland. Indeed, the Coalition recently brought forward the date for the first same-sex weddings in England and Wales to March 29.

A Scottish Government source said ministers were confident of the first ceremonies taking place by the end of the 2014 but that if all went well, "the autumn would be a reasonable time".

The Government also believes the chance of a legal challenge to the legislation is increasingly remote, given the safeguards built in to protect those unwilling to perform the ceremonies and the Catholic Church's apparent reluctance to weigh into the issue since the sex scandals involving former Cardinal Keith O'Brien.

Tuesday marks the third and final stage of the same-sex marriage bill in parliament. MSPs voted to approve the law in principle in November, when 98 voted in favour, 15 objected and five abstained, suggesting overwhelming support this week.

In Scotland, the Catholic Church and Church of Scotland oppose same-sex marriage, but a number of smaller churches and faiths are supportive, and say they would be ready to conduct same-sex religious weddings. These include the Unitarians, Quakers, Metropolitan Community Church, the Pagan Federation and Liberal Judaism.

Tom French of the Equality Network, which has campaigned for years for same-sex marriage, said: "If our MSPs pass the equal marriage bill on Tuesday, 2014 will be a milestone year for equality in Scotland. We know many couples across the country are looking forward to planning their wedding and it's great news that their big day may now come about rather sooner than expected."

A Government spokesman said: "Should the bill pass this week, we would be looking forward to the first marriages as soon as possible."

The Scottish Government's 2012 consultation on gay marriage attracted a record 77,500 responses, most of them opposed to the idea. About one-third of these were postcards and forms distributed by religious groups opposed to the change. The remainder were more evenly split. Other polling suggests most Scots support gay men and women having a right to marry.

Same-sex marriage is already legal in South Africa, Belgium, Denmark, France, Iceland, the Netherlands, Norway, Portugal, Spain, Sweden, Argentina, some parts of Mexico, Uruguay, Canada, New Zealand and some US states.